Small improvements to the contributing basics

[MeGaGiGaGon]

Description

This PR contains some small improvements to the basics about contributing that I would have liked when making my first PR.
There are also a few small changes to make formatting more consistent/add clarity/make it flow better.
This also implements a workaround for a bug in MyST-Parser causing the Next PR Number query params to be rejected.

I'm not sure why in one part of the documentation it has (.venv)$ pip install -e ".[d]" while in another (.venv)$ pip install -e .[d], and I'm not sure of the difference, so I normalized it so both use quotes.

Part of #2238.
The changes to CONTRIBUTING.md were spurred on by #4491

Since the changes are relatively minor, I've opted to not include an entry in the changelog.

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?