Rework README and CONTRIBUTING (#5429)

This shuffles sections around between README.md and CONTRIBUTING.md. CONTRIBUTING now contains information pertaining to opening PRs, README all other information. I have also moved the list of maintainers to a separate file.

I have kept most information intact for now, with two main exceptions:

I removed duplicated information.
For brevity's sake, I trimmed some explanations from the section about version checks.
I have restructured the CONTRIBUTING file to follow the order of the introductory "contribution process at a glance" section. This now serves as a bit of a table of contents.

Closes: #5422

Author: srittau