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-setup.mdx

@@ -0,0 +1,81 @@
+---
+id: js-setup
+title: Building a Desktop Plugin
+sidebar_label: Building a Desktop Plugin
+---
+import useBaseUrl from '@docusaurus/useBaseUrl';
+
+Now that we have the native side covered, let's display the data we're sending
+on the desktop side. You can check out the full workflow of building Flipper desktop
+plugins here: https://fbflipper.com/docs/extending/js-setup.html.
+
+<img alt="Custom cards UI for our sea mammals plugin" src={useBaseUrl("img/js-custom.png")} />
+
+## Dynamic Plugin loading
+
+By default, Flipper will start with the plugins it was bundled with. You can
+configure it to also look for plugins in custom directories. To do that,
+modify the `~/.flipper/config.json` file that is created the first time
+you start Flipper and add a newly created directory the `pluginPaths` attribute.
+
+Your file will then look something like this:
+
+```json
+{
+  "pluginPaths": [
+    "~/Flipper/custom-plugins/"
+  ],
+  ...
+}
+```
+
+## Creating the Plugin Package
+
+With the loading part out of the way, we can create the new plugin. For that, first
+create a new folder inside the custom plugins directory. Then use `yarn init` (`npm init` if that's more your style)
+to initialise a new JavaScript package:
+
+```bash
+$ cd ~/Flipper/custom-plugins/
+$ mkdir sea-mammals
+$ cd sea-mammals
+$ yarn init
+```
+
+Open the `package.json` and edit it. There are a few important things:
+1) "name" must start with "flipper-plugin-"
+2) "keywords" must contain "flipper-plugin"
+3) "id" must be the same as used on native side, e.g. returned by getId() method in Android plugin. In our case that is "sea-mammals".
+4) "specVersion" must contain the version of the specification according to which the plugin is defined.  Currently, Flipper supports plugins defined by the specification version 2, while version 1 is being deprecated.
+5) "title" and "icon" are optional fields specifying the plugin item appearance in the Flipper sidebar.
+
+For instance:
+
+```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"
+  },
+  "peerDependencies": {
+    "flipper": "latest"
+  },
+  "devDependencies": {
+    "flipper": "latest",
+    "flipper-pkg": "latest"
+  }
+}
+```
+*See [package.json](https://github.com/facebook/flipper/blob/master/desktop/plugins/seamammals/package.json)*
+
+Now that our package has been set up, we are ready to build a UI for our plugin. Either by using a standardized table-based plugin, or by creating a custom UI.