restructure the contributing guide

From #4355

@max-sixty:

Stepping back on the contributing doc — I admit I haven't look at it in a while — I wonder whether we can slim it down a bit, for example by linking to other docs for generic tooling — I imagine we're unlikely to have the best docs on working with GH, for example. Or referencing our PR template rather than the (now out-of-date) PR checklist.

We could also add a docstring guide since the numpydoc guide does not cover every little detail (for example, default notation, type spec vs. type hint, space before the colon separating parameter names from types, no colon for parameters without types, etc.)

Start by locating the contributing guide and comparing its PR checklist references with the current PR template. Review the existing numpydoc guidance and identify the project-specific docstring conventions listed in the issue. Done means a slimmer, current contributing guide plus a project docstring guide covering those conventions, with links replacing generic or outdated material.