Migrate website to Docusaurus 2

Summary:
Docusaurus 2 is quite a lot more powerful than docu 1 it turns out.
This should convert the website fully.

* [done] Go through migration guide https://v2.docusaurus.io/docs/migrating-from-v1-to-v2
* [done] Convert landing page html
* [done] Convert all images to img tags
* [done] Convert all .md files to .mdx
* [done] Make sure ui-doc generation and including still works
* [done] Scan every page visually for sanity check
* [done] Make sure footer still works
* [done] Make sure search still works
* [done] Change all links/ to links/index
* [done] Change all links.md to links
* [done] Add some custom css to make the navbar look like the old one and darken the footer.

Reviewed By: passy

Differential Revision: D21158717

fbshipit-source-id: 5f45b711b1b6fd5ece4c5c15c55635c7ebbfb568

docs/features/navigation-plugin.mdx

@@ -0,0 +1,15 @@
+---
+id: navigation-plugin
+title: Navigation
+---
+import useBaseUrl from '@docusaurus/useBaseUrl';
+
+→ [See setup instructions for the Navigation Plugin](setup/navigation-plugin)
+
+The Navigation Plugin allows users to quickly navigate to deep links within their mobile applications to help speed up the development cycle. The plugin is designed to integrate easily within your existing navigation framework or as a stand alone tool. Users can bookmark deep links and jump to them via the button in the tool bar.
+
+<img alt="Navigation Plugin Button" src={useBaseUrl("img/navigation-plugin-1.png")} />
+
+Navigation events within the app can also be logged to Flipper. This allows the user to view past navigation events and jump straight to them, or export the navigation events for reporting.
+
+<img alt="Navigation Plugin UI" src={useBaseUrl("img/navigation-plugin-2.png")} />