[2.0.0-rc] Update documentation

@pdureau there is this documentation here (which I used to duplicate on d.org) : https://ui-patterns.readthedocs.io/en/8.x-1.x/

Also, lets take some Support Requests to make a FAQ (Frequently Asked Questions) : https://www.drupal.org/project/issues/ui_patterns?text=&status=Open&prio... →

Related issue: 💬 Documentation upgrade from v1 to v2 Active

I take it

Doing some tests with mkdocs, so the current issue will also cover 📌 [2.0.0-beta3] Move specs to markdown in git repo Active