Combine internal and external navigation plugin setup pages

Summary:
Now in public, the setup page remains unchanged, well, the headings are one level smaller.

In internal, the page is divided into two sections: for buck based apps, and non-buck based ones.

Reviewed By: mweststrate

Differential Revision: D25565953

fbshipit-source-id: abf4316a28970cd93d0cc9b56dd707f00a674a48

website/sidebars.js

@@ -27,7 +27,6 @@ module.exports = {
       'features/react-native',
       ...fbInternalOnly([
         'fb/Memory-Tools',
-        'fb/Navigation-Plugin',
         'fb/supporting-feed-inspector',
         'fb/sections',
         'fb/Trace',