Update Python Packaging section to use `pyproject.toml` instead of `setup.cfg`

[sjvrijn]

Currently, the Making an installable package section of the Python guide still refers to setup.cfg as the main of two required files.

Since we've also moved away from setup.cfg in our python cookiecutter template, I think we should update the guide to match what we've currently implemented as best practice?

[egpbos]

In theory (and in practice, see e.g. https://github.com/NLeSC/python-template/issues/328), we could also rewrite the setuptools parts a bit to make people aware of alternatives that we started using, i.e. hatch/hatchling.

[bouweandela]

Maybe we could replace that whole section by one or more links to tutorials/documentation pages? The topic is quite generic, so I would expect that good resources are available, and the current text is rather detailed and therefore likely to go out of date quickly.

[egpbos]

Sounds good to me.

[ofek]

Let me know if I can help with reviews (I maintain Hatch/Hatchling)!

[egpbos]

@sjvrijn Do you want to make a PR for this? :)