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/tutorial/intro.mdx

@@ -0,0 +1,20 @@
+---
+id: intro
+title: Intro
+---
+import useBaseUrl from '@docusaurus/useBaseUrl';
+
+<img alt="Android App Tutorial" src={useBaseUrl("img/android-tutorial.png")} />
+
+In this tutorial, we show you how how easy it is to build a Flipper plugin
+for Android and iOS that extracts data from your native application and
+displays it in the desktop app.
+
+We then guide you through the process of converting a basic table plugin into
+a full plugin with custom UI components.
+
+Before we get started, let's define two terms we will use frequently throughout
+this tutorial:
+
+- **Desktop app**: This is the Electron-based application you run on your desktop.
+- **Mobile client**: This is the mobile app running most likely on a phone or other mobile device. It connects to the desktop app.