Unify documentation on how to get a plugin instance

Summary: There were 3 pages describing how to obtain a plugin instance, public, ios and android. iOS didn't differ from the public one, and android only in the fact that internally DI is available. So combined the pages with just an optional section for DI inside FB on Android.

Reviewed By: jknoxville

Differential Revision: D25588057

fbshipit-source-id: 3a54ae699130a4c5ba018220708f844a35a8d6a9

website/sidebars.js

@@ -101,18 +101,6 @@ module.exports = {
       'extending/arch',
       'extending/client-plugin-lifecycle',
       'extending/layout-inspector',
-      ...fbInternalOnly([
-        {
-          Android: [
-            'fb/android-plugin-development-Android-interacting-0',
-          ],
-        },
-        {
-          iOS: [
-            'fb/ios-plugin-development-sending-data-to-an-ios-plugin-0',
-          ],
-        },
-      ]),
     ],
     Workflow: [
       'extending/js-setup',