Remove most of ExoPlayer page, replacing it with links to ExoPlayer site.

- The dev guide has moved to Github, where we can easily maintain it
  together with the code.
- I'd like us to still keep this page on the Android site, to direct
  developers toward using ExoPlayer.
- I have no idea what branch this should be on, but can easily cherry-pick
  to another one as required.

Change-Id: I9179b822988698053463eed7162e970cab3e1ae4