Update contribution guidelines

[pekkaklarck]

The most important task related to this is explaining how to use automatic code formatting (#5387). At the same time, it makes sense to go through the guidelines in general and see are they up-to-date and can they be enhanced. Based on PR #5375 removing Python 2 references, it is possible that there's some old cruft that should be updated or removed.

[pekkaklarck]

From #5387:

Update contribution guidelines. Most importantly, instruct contributors to run invoke format before creating a PR. Should also add instructions when to use the "magic comma" or # fmt: skip to avoid suboptimal automatic formatting, but this should be an optional task for contributors.

The aforementioned cases where automatic formatting is suboptimal are discussed in #5387 in comments having titles like "Problem x: Explanation". Problem 3 is handled automatically, but problems 1, 2, 4 and 5 need special treatment.