Update some more "contributing" docs
Summary:
Ref T12922.
- Remove most mentions to "Contributing Feature Requests".
- Raise the barrier to entry on code contributions.
I'm going to tweak "Bug Reports" in a followup to be more similar to "Feature Requests", but that's a slightly more involved change.
Test Plan: Read new docs.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12922
Differential Revision: https://secure.phabricator.com/D18235