[Loloekk]

Summary

Puts the generated family pages into the sidebar and replaces the old hand-written tree.

• New two-level sidebar: Fundamentals · React Containers · React Components & Modals ·
  React Common Types · Core Containers · Core Components & Modals · Guides · Meta. A family
  (Stack, Tabs, …) is one page; its table of contents is the in-page navigation.
• React Common Types is a single link styled as a section (.sidebar-section-link): it
  shares the section labels' font rule in typography.css, and its spacing comes from
  t-rex-ui's label and section-list margins (named variables, with the mobile value), so the
  gaps between all sections are equal.
• Fundamentals, Guides, Meta and Core in the new shape — pages and parts named *-empty
  until they get content or be removed. Introduction keeps slug: /, so /docs/ still resolves.
• The documentation guidelines and the MDX components cheatsheet (both by Szymon Halski)
  move into Meta → Contributing as its "Documentation guidelines" and "MDX components"
  sections; only headings and links were adjusted.
• Removed: docs/components, docs/navigators, docs/experimental, docs/react-navigation,
  docs/contributing, the old Fundamentals and Guides pages, and the two sidebar CSS rules
  only they used.
• Breadcrumbs on; the navbar "Docs" link and the home page button point at Getting Started.

Known: stub pages still carry -empty in their URLs (e.g. /docs/fundamentals/getting-started-empty); to be changed separately.

Test plan

• yarn build passes locally
• Visual check at http://localhost:3000/react-native-screens/

Screenshots

[kligarski]

1. I really don't like that "React Common Types" is a link but other same-level headings in the sidebar are not links. I'm wondering if artifically adding a subpage wouldn't be better:

React Common Types
- Types

or something like this.

2. Nit: "overview" placeholder for Core Components/Modals looks odd. Let's maybe add placeholders for some of the real components instead. You can also add placeholders for stack and split in core containers section.

3. In-page navigation is a bit awkward still but I'm guessing that this will be handled separately with other page layout PR.

[Loloekk]

1. Not sure yet which way is better. It's a small change, so let's decide later.
2. I removed the Core sections for now, since nothing will go there yet. They'll come back once the native API pages have content.
3. Yes, the table of contents will change later.