docs: add the start of a contributor's guide

[JohnVillalovos]

Create an initial contributor's guide that will hopefully be expanded
on in the future.

[nejch]

Thanks a lot @JohnVillalovos! Just one initial concern I have in general is that this somewhat duplicates https://github.com/python-gitlab/python-gitlab/blob/main/CONTRIBUTING.rst. Can we combine these somehow? the root-level one should probably be more concise and link to more details I guess, but have enough info for a quickstart.

For example, I noticed the same kind of duplication with README.rst and our install instructions and intro etc, so I ended up just linking the repo file in the docs. Would that make sense to avoid having to update things twice?