Document PACK download mechanism

Summary:
This is a first stab at documenting the new core distribution mechanism.
There are definitely areas I just brush over that should be expanded further,
but hopefully the release process and testing steps at the end help to clarify
some of the issues we've seen recently.

Reviewed By: mweststrate

Differential Revision: D23453713

fbshipit-source-id: 9efcfca1fbdbf5606080510c3e80bd19b49caf10

website/sidebars.js

@@ -102,6 +102,9 @@ module.exports = {
       'extending/arch',
       'extending/layout-inspector',
       'extending/testing-rn',
+      ...FBInternalOnly([
+        'extending/fb/launcher',
+      ]),
     ],
   },
   'fb-internal': {