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/android-native.mdx

@@ -108,7 +108,7 @@ dependencies {
 
 ## Enabling plugins
 
-Finally, you need to add plugins to your Flipper client. Above, we have only added the Layout Inspector plugin to get you started. See [Network Plugin](setup/network-plugin.md) and [Layout Inspector Plugin](setup/layout-plugin.md) for information on how to add them, and also enable Litho or ComponentKit support. You can check the sample apps in the [GitHub repo](https://github.com/facebook/flipper) for examples of integrating other plugins.
+Finally, you need to add plugins to your Flipper client. Above, we have only added the Layout Inspector plugin to get you started. See [Network Plugin](setup/network-plugin) and [Layout Inspector Plugin](setup/layout-plugin) for information on how to add them, and also enable Litho or ComponentKit support. You can check the sample apps in the [GitHub repo](https://github.com/facebook/flipper) for examples of integrating other plugins.
 
 ## Having trouble?
 

docs/getting-started/index.mdx

@@ -2,6 +2,7 @@
 id: index
 title: Desktop App
 ---
+import useBaseUrl from '@docusaurus/useBaseUrl';
 
 Flipper helps you debug Android and iOS apps running in an emulator/simulator or connected physical development devices. Flipper consists of two parts:
 
@@ -16,4 +17,4 @@ The desktop part of Flipper doesn't need any particular setup. Simply download t
 
 Once you start Flipper and launch an emulator/simulator or connect a device, you will already be able to see the device logs in Flipper. To see app specific data, you need to integrate our native SDKs with your app.
 
-![Logs plugin](assets/initial.png)
+<img alt="Logs plugin" src={useBaseUrl("img/initial.png")} />

docs/getting-started/ios-native.mdx

@@ -223,7 +223,7 @@ class AppDelegate: UIResponder, UIApplicationDelegate {
 
 ## Enabling plugins
 
-Finally, you need to add plugins to your Flipper client. Above, we have only added the Layout Inspector plugin to get you started. See [Network Plugin](setup/network-plugin.md) and [Layout Inspector Plugin](setup/layout-plugin.md) for information on how to add them, and also enable Litho or ComponentKit support. You can check the sample apps in the [GitHub repo](https://github.com/facebook/flipper) for examples of integrating other plugins.
+Finally, you need to add plugins to your Flipper client. Above, we have only added the Layout Inspector plugin to get you started. See [Network Plugin](setup/network-plugin) and [Layout Inspector Plugin](setup/layout-plugin) for information on how to add them, and also enable Litho or ComponentKit support. You can check the sample apps in the [GitHub repo](https://github.com/facebook/flipper) for examples of integrating other plugins.
 
 ## Having trouble?