Add a route provider and controller for add page/form.

entity.module

+<?php
+
+/**
+ * @file
+ * Provides expanded entity APIs.
+ */
+
+use Drupal\Core\Url;
+
+/**
+ * Implements hook_theme().
+ */
+function entity_theme() {
+  return [
+    'entity_add_list' => [
+      'variables' => [
+        'bundles' => [],
+        'bundle_type' => NULL,
+        'form_route_name' => NULL,
+      ],
+      'template' => 'entity-add-list',
+    ],
+  ];
+}
+
+/**
+ * Prepares variables for the list of available bundles.
+ *
+ * Default template: entity-add-list.html.twig.
+ *
+ * @param array $variables
+ *   An associative array containing:
+ *   - bundle_type: The entity type of the bundles.
+ *   - bundles: An array of bundles.
+ *   - form_route_name: The add form route.
+ */
+function template_preprocess_entity_add_list(&$variables) {
+  $bundle_type = \Drupal::entityTypeManager()->getDefinition($variables['bundle_type']);
+  $variables += [
+    'class' => str_replace('_', '-', $bundle_type->getBundleOf()) . '-add-list',
+    'create_bundle_url' => Url::fromRoute('entity.' . $bundle_type->id() . '.add_form'),
+    'bundle_type_label' => $bundle_type->getLowercaseLabel(),
+  ];
+
+  foreach ($variables['bundles'] as $bundle) {
+    $url = Url::fromRoute($variables['form_route_name'], [$bundle->getEntityTypeId() => $bundle->id()]);
+    $variables['bundles'][$bundle->id()] = [
+      'bundle' => $bundle->id(),
+      'add_link' => \Drupal::l($bundle->label(), $url),
+      'description' => [
+        '#markup' => $bundle->getDescription(),
+      ],
+    ];
+  }
+}

src/Controller/EntityCreateController.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity\Controller\EntityCreateController.
+ */
+
+namespace Drupal\entity\Controller;
+
+use Drupal\Core\Controller\ControllerBase;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Render\RendererInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Url;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * A generic controller for creating entities.
+ */
+class EntityCreateController extends ControllerBase {
+
+  /**
+   * The entity type bundle info.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeBundleInfoInterface
+   */
+  protected $entityTypeBundleInfo;
+
+  /**
+   * The renderer service.
+   *
+   * @var \Drupal\Core\Render\RendererInterface
+   */
+  protected $renderer;
+
+  /**
+   * Constructs a new EntityCreateController object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
+   *   The entity type bundle info.
+   * @param \Drupal\Core\Render\RendererInterface $renderer
+   *   The renderer.
+   */
+  public function __construct(EntityTypeBundleInfoInterface $entity_type_bundle_info, RendererInterface $renderer) {
+    $this->entityTypeBundleInfo = $entity_type_bundle_info;
+    $this->renderer = $renderer;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('entity_type.bundle.info'),
+      $container->get('renderer')
+    );
+  }
+
+  /**
+   * Displays add links for the available bundles.
+   *
+   * Redirects to the add form if there's only one bundle available.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request.
+   *
+   * @return \Symfony\Component\HttpFoundation\RedirectResponse|array
+   *   If there's only one available bundle, a redirect response.
+   *   Otherwise, a render array with the add links for each bundle.
+   */
+  public function addPage($entity_type_id, Request $request) {
+    $entity_type = $this->entityTypeManager()->getDefinition($entity_type_id);
+    $bundle_type = $entity_type->getBundleEntityType();
+    $form_route_name = 'entity.' . $entity_type_id . '.add_form';
+    $build = [
+      '#theme' => 'entity_add_list',
+      '#cache' => [
+        'tags' => $entity_type->getListCacheTags(),
+      ],
+      '#bundle_type' => $bundle_type,
+      '#form_route_name' => $form_route_name,
+    ];
+    $bundles = array_keys($this->entityTypeBundleInfo->getBundleInfo($entity_type_id));
+    // Filter out the bundles the user doesn't have access to.
+    $access_control_handler = $this->entityTypeManager()->getAccessControlHandler($bundle_type);
+    foreach ($bundles as $index => $bundle_name) {
+      $access = $access_control_handler->createAccess($bundle_name, NULL, [], TRUE);
+      if (!$access->isAllowed()) {
+        unset($bundles[$index]);
+      }
+      $this->renderer->addCacheableDependency($build, $access);
+    }
+    // Redirect if there's only one bundle available.
+    if (count($bundles) == 1) {
+      $bundle_name = reset($bundles);
+      return $this->redirect($form_route_name, [$bundle_type => $bundle_name]);
+    }
+    // The theme function needs the full bundle entities.
+    $build['#bundles'] = $this->entityTypeManager->getStorage($bundle_type)->loadMultiple($bundles);
+
+    return $build;
+  }
+
+  /**
+   * The title callback for the add page.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   *
+   * @return string
+   *   The page title.
+   */
+  public function addPageTitle($entity_type_id) {
+    $entity_type = $this->entityTypeManager()->getDefinition($entity_type_id);
+    return $this->t('Add @entity-type', ['@entity-type' => $entity_type->getLowercaseLabel()]);
+  }
+
+  /**
+   * Provides the add form for an entity of a specific bundle.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The route match.
+   *
+   * @return array
+   *   The add form.
+   */
+  public function addForm($entity_type_id, RouteMatchInterface $route_match) {
+    $entity_type = $this->entityTypeManager()->getDefinition($entity_type_id);
+    $bundle_type = $entity_type->getBundleEntityType();
+    $bundle_key = $entity_type->getKey('bundle');
+    $bundle_name = $route_match->getRawParameter($bundle_type);
+    $entity = $this->entityTypeManager()->getStorage($entity_type_id)->create([
+      $bundle_key => $bundle_name,
+    ]);
+
+    return $this->entityFormBuilder()->getForm($entity, 'add');
+  }
+
+  /**
+   * The title callback for the add form.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The route match.
+   *
+   * @return string
+   *   The page title.
+   */
+  public function addFormTitle($entity_type_id, RouteMatchInterface $route_match) {
+    $entity_type = $this->entityTypeManager()->getDefinition($entity_type_id);
+    $bundle_type = $entity_type->getBundleEntityType();
+    $bundle_name = $route_match->getRawParameter($bundle_type);
+    $bundles = $this->entityTypeBundleInfo->getBundleInfo($entity_type_id);
+    if (count($bundles) == 1) {
+      $title = $this->t('Add @entity-type', ['@entity-type' => $entity_type->getLowercaseLabel()]);
+    }
+    else {
+      $title = $this->t('Add @bundle', ['@bundle' => $bundles[$bundle_name]['label']]);
+    }
+
+    return $title;
+  }
+
+}

src/Routing/CreateUIRouteProvider.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity\Routing\CreateUIRouteProvider.
+ */
+
+namespace Drupal\entity\Routing;
+
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Entity\Routing\EntityRouteProviderInterface;
+use Symfony\Component\Routing\Route;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Provides HTML routes for creating entities.
+ *
+ * This class provides the following routes for entities, with title callbacks:
+ * - add-page
+ * - add-form
+ */
+class CreateUIRouteProvider implements EntityRouteProviderInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRoutes(EntityTypeInterface $entity_type) {
+    $routes = new RouteCollection();
+    if ($route = $this->addPageRoute($entity_type)) {
+      $routes->add('entity.' . $entity_type->id() . '.add_page', $route);
+    }
+    if ($route = $this->addFormRoute($entity_type)) {
+      $routes->add('entity.' . $entity_type->id() . '.add_form', $route);
+    }
+
+    return $routes;
+  }
+
+  /**
+   * Returns the add page route.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
+   *   The entity type.
+   *
+   * @return \Symfony\Component\Routing\Route|null
+   *   The generated route, if available.
+   */
+  protected function addPageRoute(EntityTypeInterface $entity_type) {
+    if ($entity_type->hasLinkTemplate('add-page')) {
+      $route = new Route($entity_type->getLinkTemplate('add-page'));
+      $route->setDefault('_controller', '\Drupal\entity\Controller\EntityCreateController::addPage');
+      $route->setDefault('_title_callback', '\Drupal\entity\Controller\EntityCreateController::addPageTitle');
+      $route->setDefault('entity_type_id', $entity_type->id());
+      $route->setRequirement('_entity_create_access', $entity_type->id());
+      return $route;
+    }
+  }
+
+  /**
+   * Returns the add form route.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
+   *   The entity type.
+   *
+   * @return \Symfony\Component\Routing\Route|null
+   *   The generated route, if available.
+   */
+  protected function addFormRoute(EntityTypeInterface $entity_type) {
+    if ($entity_type->hasLinkTemplate('add-form')) {
+      $bundle_type = $entity_type->getBundleEntityType();
+      $route = new Route($entity_type->getLinkTemplate('add-form'));
+      $route->setDefault('_controller', '\Drupal\entity\Controller\EntityCreateController::addForm');
+      $route->setDefault('_title_callback', '\Drupal\entity\Controller\EntityCreateController::addFormTitle');
+      $route->setDefault('entity_type_id', $entity_type->id());
+      $route->setOption('parameters', [
+        $bundle_type => ['type' => 'entity:' . $bundle_type],
+      ]);
+      $route->setRequirement('_entity_create_access', $entity_type->id());
+      return $route;
+    }
+  }
+
+}

templates/entity-add-list.html.twig

+{#
+/**
+ * @file
+ * Default theme implementation to present a list of available bundles.
+ *
+ * Available variables:
+ *   - class: The entity type specific class. E.g. 'node-add-list'.
+ *   - create_bundle_url: The url to the bundle creation page.
+ *   - bundle_type_label: The lowercase label of the bundle entity type.
+ *   - bundles: A list of bundles, each with the following properties:
+ *     - add_link: link to create an entity of this bundle.
+ *     - description: Bundle description.
+ *
+ * @see template_preprocess_entity_add_list()
+ *
+ * @ingroup themeable
+ */
+#}
+{% if bundles is not empty %}
+  <dl class="entity-add-list {{ class }}">
+    {% for bundle in bundles %}
+      <dt>{{ bundle.add_link }}</dt>
+      <dd>{{ bundle.description }}</dd>
+    {% endfor %}
+  </dl>
+{% else %}
+  <p>
+    {% trans %}
+      Go to the <a href="{{ create_bundle_url }}">{{ bundle_type_label }} creation page</a> to add a new {{ bundle_type_label }}.
+    {% endtrans %}
+  </p>
+{% endif %}