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/databases-plugin.mdx

@@ -0,0 +1,20 @@
+---
+id: databases-plugin
+title: Databases
+---
+import useBaseUrl from '@docusaurus/useBaseUrl';
+
+→ [See setup instructions for the Databases plugin](setup/databases-plugin)
+
+The Databases plugin provides developers with read-write access to their databases:
+- browse your tables' data
+- see your tables' structure
+- execute queries
+- mark queries as favorites
+- see logs of past executed queries
+
+
+<img alt="Databases Plugin 1" src={useBaseUrl("img/databases-plugin-1.png")} />
+<img alt="Databases Plugin 2" src={useBaseUrl("img/databases-plugin-2.png")} />
+
+Note: this plugin is only available for Android.