Write bug report, feature request and code contribution guides

Summary:
Fixes T6347. This refines the "contributor guide" documents to basically lock down support further. Notable changes in policy:

  - Bugs: Emphasis on reproduction steps, strong emphasis on using Maniphest. Emphasis on what we support.
  - Features: Emphasis on describing problems instead of solutions, emphasis on realistic expecations about timelines. Strong emphasis on using Maniphest.
  - Code: Strong emphasis on coordinating with us first. No GitHub pull requests. Emphasis on us ignoring contributions we don't have time to deal with. Suggests local forks.

Test Plan: Read these through; let me generate them and take some screenshots for easier reading.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6347

Differential Revision: https://secure.phabricator.com/D10764