Review contributor documentation

[andreww]

Currently we have documentation on contribution in two palaces:

• https://github.com/GreenScheduler/cats/blob/main/CONTRIBUTING.md
• https://cats.readthedocs.io/en/latest/contributing.html#contributing-code-or-documentation

Should probably make the first point more obviously to the second (and maybe briefly summarise), and make the read the docs version easer to read. We don't cover things like:

• Use (or not) or declaration of AI use
• Attribution (for licensing)
• The need for testing

etc.

[andreww]

Consider PR templates too

[andreww]

Do we need something like this: https://en.wikipedia.org/wiki/Developer_Certificate_of_Origin