Cleanup and reorganize docs and tooling for contributors and maintainers

This pull request cleans up and reorganizes hack/. It doesn't affect how the dev and build tools work. It's mostly organizing and updating existing docs, and writing new ones.

* Remove a bunch of unmaintained scripts and docs.
* Start MAINTAINERS.md as a reference for maintainers. Needs extra work!
* Link CONTRIBUTORS.md to CONTRIBUTING.md in the root.
* Change or at least remove out-of-date content from release checklist. There are still some FIXMEs in there.