DOC Adding more details on how to contribute to the library

[rcap107]

Very early and rough draft for some improvements to the page on how to contribute to skrub. Follow-up on #1130, and still very much a work in progress.

I added some of the points that I mentioned in the issue, and some examples. As I already said, this is an early draft that we could use for discussing what should be added, and wording and stuff.

I would like to add something more on writing doctests and maybe be more specific when it comes to CI, but I don't know enough about either do do that 😅

A checklist of the operations that should be performed for the PR (pre-commit, write tests, update the changelog etc.)

• [x] How to run tests using a specific environment with pixi on the local machine for debugging
• [x] How to check for code coverage on the local machine
• [x] How to check doctests
• [x] Some information on where the code for testing is, and how to structure tests
• [ ] The documentation style to use (numpydoc)

[jeromedockes]

I think this is already a clear improvement so I'll merge it and we can have other prs to further improve some sections if we want