Move internal plugin pages to where the public ones are
Move internal plugin pages to where the public ones are

Summary:
^

The navigation plugin also includes setup instructions which will be moved to it's own page next.
There are also now 2 navigation plugin feature pages. These still need to be consolidated.

Reviewed By: passy

Differential Revision: D25564150

fbshipit-source-id: 1fe0d853f6fa94ec62b39edacd268519fa18485a

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper