Add some basic SDC

Modern Drupal themes should be constructed on Single Directory Components. It makes theme following the modular patterns known from other well working ecosystems, like React, and is crucial for Canvas module.

As a theme developer, i expect starterkit to have components directory with some basic SDC, wich are used literally on every project.

Here are the candidates:

• Section - includes centering container with max width, paddings and definable background, can be used also in page.html.twig
• Rich text - Tailwind prose class wrapper, ideal for rendering the content from WYSiWYG fields.