Click-specific contribution guidelines

[kdeldycke]

Here is the list of contribution guidelines I accumulated over the past year based on feedbacks from the community.

My plan is to:

• collect feedback on them to refine/expand them
• decides which should be scoped as Click-specific
• decides on which should be contributed back to the Pallets umbrella at https://github.com/pallets/.github/blob/main/CONTRIBUTING.md
• look into the current state of stable before 8.3.3 release and applies them thoroughly

This PR is a follow-up on #3151 , which inaugurated the creation of Click-specific docs/contributing.md document. I will rebase this PR once the later is merged.

[davidism]

Check out https://palletsprojects.com/contributing/ first. Most of this either is mentioned there, or should be expanded there.

For example, the changelog is already mentioned in https://palletsprojects.com/contributing/pr

Also, for docs, I'm sorry I keep harping on this, but it's not that GitHub links don't belong in docs but in comments. It's that the links should not be anywhere. The docs and comments must stand on their own. Do not include GitHub issue/pr numbers or links. The only place issues or PRs should be linked or mentioned is in the changelog page. This applies to Click issues/prs, you can still add links to cpython or other projects, if the code is working around a reported issue in something else.