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/getting-started/react-native.mdx

@@ -0,0 +1,35 @@
+---
+id: react-native
+title: Set up your React Native App
+sidebar_label: React Native
+---
+
+Starting with React Native 0.62, after generating your project with `react-native init`, the Flipper integration is ready out of the box for debug builds:
+
+* For Android, start the Flipper Desktop application, and start your project using `yarn android`. Your application will appear in Flipper.
+* For iOS, run `pod install` once in the `ios` directory of your project. After that, run `yarn ios` and start Flipper. Your application will show up in Flipper.
+
+By default, the following plugins will be available:
+
+* Layout Inspector
+* Network
+* Databases
+* Images
+* Shared Preferences
+* Crash Reporter
+* React DevTools
+* Metro Logs
+
+Additional plugins can be installed through the plugin manager.
+
+To create your own plugins and integrate with Flipper using JavaScript, check out our [writing plugins for React Native](/docs/tutorial/react-native) tutorial!
+
+To update the Flipper SDK in your project, you bump the version in the `ios/Podfile` and `android/gradle.properties` files of your project. 
+
+## Manual Setup
+
+If you are not using a default React Native template or cannot use the upgrade tool,
+you can find instructions for how to integate Flipper into your projects in these guides:
+
+- [React Native for Android](/docs/getting-started/react-native-android)
+- [React Native for iOS](/docs/getting-started/react-native-ios)