Move plugin docs to plugin folders

Summary: Moved plugin documentation and related assets to plugin folders, fixed links and configured redirects where required. Now these docs are used for both showing docs in Flipper and generating Flipper docs website.

Reviewed By: passy

Differential Revision: D29465567

fbshipit-source-id: 3ec4240b215b0d5baea5154f64266a9ba7ead3a5

desktop/plugins/public/network/docs/setup.mdx

@@ -1,5 +1,6 @@
 import Tabs from '@theme/Tabs';
 import TabItem from '@theme/TabItem';
+import ProtobufRetrofitSetup from './protobuf-retrofit.mdx';
 
 To use the network plugin, you need to add the plugin to your Flipper client instance.
 
@@ -94,3 +95,7 @@ client?.add(FlipperKitNetworkPlugin(networkAdapter: SKIOSNetworkAdapter()))
 
 </TabItem>
 </Tabs>
+
+## Protobut + Retrofit Setup
+
+<ProtobufRetrofitSetup />