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/js-publishing.mdx

@@ -0,0 +1,66 @@
+---
+id: js-publishing
+title: Publishing your Plugin
+sidebar_label: Publishing
+---
+import useBaseUrl from '@docusaurus/useBaseUrl';
+
+Once you're happy with your plugin and want the world to see it,
+you can publish it to npm. Ensure that your plugin follows these
+two rules:
+
+- The package name should to start with "flipper-plugin-". This makes
+  it easier to see what the purpose of the package is.
+- The package must include the keyword "flipper-plugin".
+- Source code of the plugin must be bundled by "flipper-pkg" tool.
+
+A valid example `package.json` could look like this:
+
+```json
+{
+  "name": "flipper-plugin-sea-mammals",
+  "id": "sea-mammals",
+  "specVersion": 2,
+  "version": "2.0.0",
+  "main": "dist/bundle.js",
+  "flipperBundlerEntry": "src/index.tsx",
+  "license": "MIT",
+  "keywords": ["flipper-plugin"],
+  "icon": "apps",
+  "title": "Sea Mammals",
+  "category": "Example Plugin",
+  "scripts": {
+    "prepack": "flipper-pkg bundle"
+  },
+  "dependencies": {
+    "flipper": "latest"
+  },
+  "devDependencies": {
+    "flipper-pkg": "latest"
+  }
+}
+```
+
+When you have confirmed that your `package.json` is correct,
+run `yarn publish` or `npm publish` and follow the instructions.
+
+## Installing Plugins
+
+Once your plugin is published you can find it, alongside other
+available Flipper plugins, by clicking on "Manage Plugins..."
+in the bottom of the left sidebar and selecting the
+"Install Plugins" tab. It may take a few moments for the
+search index to update and your plugin to appear.
+
+<img alt="Install plugins" src={useBaseUrl("img/install-plugins.png")} />
+
+## Native Distribution
+
+Depending on whether the client-side part of your plugin targets
+Android, iOS or React Native, we recommend you use the standard
+package distribution mechanism for the platform.
+
+This may be Maven Central, JCenter or GitHub Packages for Android,
+CocoaPods for iOS and npm or GitHub Packages for React Native.
+Make sure to leave setup instructions in the README of your
+npm package.