ISTWCMS-5595:site container css intial blocks

src/patterns/01-core/props/_colors.scss

src/patterns/01-core/props/_fonts.scss

src/patterns/01-core/props/_zindex.scss

src/patterns/03-layouts/_index.scss

src/patterns/03-layouts/content/_content.scss

src/patterns/03-layouts/content/content.md

src/patterns/03-layouts/content/content.twig

src/patterns/03-layouts/footer/_footer.scss

src/patterns/03-layouts/footer/footer.md

src/patterns/03-layouts/footer/footer.twig

src/patterns/03-layouts/footer/footer.yml

src/patterns/03-layouts/header/_header.scss

src/patterns/03-layouts/header/header.md

src/patterns/03-layouts/header/header.twig

src/patterns/03-layouts/header/header.yml

src/patterns/03-layouts/layout/_layout.scss

src/patterns/03-layouts/region/_region.scss

src/patterns/03-layouts/site-container/_site-container.scss

src/patterns/03-layouts/site-container/site-container.md

src/patterns/03-layouts/site-container/site-container.twig