[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

Current progress:

$ tree docs/ | grep -v png
├── 1-introduction.md 
├── 1-users
│   ├── 0-component-form.md
│   ├── 1-as-block.md
│   ├── 2-as-layout.md
│   ├── 3-in-field-formatter.md
│   ├── 4-with-views.md
│   ├── 5-presenter-templates.md 
├── 2-authors
│   ├── 0-authoring-a-component.md
│   ├── 1-stories-and-library.md
│   ├── 2-best-practices.md
│   ├── 3-best-practices_sdc_devel.md
│   ├── 4-migration-from-UIP1.md
├── 2-frequently-asked-questions.md
├── 3-devs
│   ├── 1-source-plugins.md
│   ├── 2-prop-type-plugins.md
│   ├── 3-internals.md