SDK / exoplayer

 - The ExoPlayer docs will link to the media3 ones (for now, we can
   permanently redirect once no further stand-alone ExoPlayer release
   is published).
 - Contibuting issues and PRs should only happen on the media3 repo.
 - The media3 README is updated to remove any pre-release status
   information.

PiperOrigin-RevId: 516234594
(cherry picked from commit 8a9c51a8)