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