Add readme for SkipToSelectedBlock component

[mikachan]

Description

This PR adds a README.md as documentation for the SkipToSelectedBlock component (see #22891). I used the original PR for this component as a reference.

How has this been tested?

This change includes documentation only.

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).

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @mikachan! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[mkaz]

Great, thanks so much for improving the documentation 👍
All looks good and makes sense.