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.