Add information about skip links to the block theme overview

[carolinan]

Description

Adds short documentation for the skip link feature implemented with #30336.
Closes #34870.

Types of changes

Documentation

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[mkaz]

This looks good 👍

Thanks for your efforts to try to keep the documentation up to date!

[jamieschmid]

Dev notes:
Because the nav is being loaded in the content and not in a separate template part, on most pages <main> is around the navigation as well, so it has no place to link to. This was a solution for us to have the floating main navigation.

Blog single posts: works, but I had to put <main> around the body copy only so it skips the post title
Blog categorie pages: works, but missing page title because same reason