Link to Discuss as a lightweight discussion forum (#20697)

Summary:
kelset should we link to Discuss more prominently? There are some lightweight discussions that probably don't belong in the discussions repo.
Pull Request resolved: https://github.com/facebook/react-native/pull/20697

Differential Revision: D9386345

Pulled By: hramos

fbshipit-source-id: 53d81bcc3d9b2570a094017cc134726140e1a3bb