Improving the section on how to contribute

[rcap107]

Describe the issue linked to the documentation

Hello, while I was working on my first PR I hit a few snags and problems that may also happen to new contributors, so I thought it might be useful to update the section in the documentation relative to contributing to skrub.

Some of the things I would like to add:

• A checklist of the operations that should be performed for the PR (pre-commit, write tests, update the changelog etc.)
• How to run tests using a specific environment with pixi on the local machine for debugging
• How to check for code coverage on the local machine
• How to check doctests
• Preparing and running pre-commit
• Some information on where the code for testing is, and how to structure tests

If you have more ideas, please add them!

@Vincent-Maladiere suggested having a section on working with pixi, but I don't have enough experience with it to write it.

Suggest a potential alternative/fix

No response

[jeromedockes]

thanks @rcap107 that's a great idea. I guess this is the file to update: https://github.com/skrub-data/skrub/blob/main/CONTRIBUTING.rst