Move fb/plugins to Features section

Summary:
Moving this file outside of the "FB Internal section" so we can get rid of it.

I think this file can probably be replaced by the "flipper marketplace" at some point, driven programatically by package.json files etc, but I'm not sure if such a page exists at the moment.

Reviewed By: passy

Differential Revision: D25588707

fbshipit-source-id: 0809c21b30410252ef146cfcd161b836849dbbf0

website/sidebars.js

@@ -31,6 +31,7 @@ module.exports = {
         'fb/sections',
         'fb/Trace',
         'fb/mobile-config',
+        'fb/plugins',
       ]),
     ],
   },
@@ -140,9 +141,6 @@ module.exports = {
       'fb/Add-Support-Group-to-Flipper-Support-Form',
       'fb/Help-Updating-Flipper',
       'extending/testing-rn',
-      {
-        'Internal Plugins': ['fb/plugins'],
-      },
     ]),
   },
 };