CONTRIBUTING.md improvement: fail to build docs due to depricated `imghdr`

📚 The doc issue

For my very first contribution I tried to following CONTRIBUTING.md as closely as possible but had the following UX issue:

Building the docs depends on a rather old Sphinx v5.0.0 which needs the deprecated imghdr module (Could not import extension sphinx.builders.epub3 (exception: No module named 'imghdr')). CONTRIBUTING.md however does not mention a version constraint about python. Therefore I needed to downgrade Python and go trought the whole setup process again.

Improvement suggestion:

• Either mentions at the beginning of CONTRIBUTING.md that Python <3.13 is needed
• Or update Sphynx in the docs requirements to a up to date version

In Additon (closely related to my user experience following CONTRIBUTING.md):

• It would be of great benefit having all dev dependencies (and version constraint) in one or several requirements files for easier setup
• Currently, the dependencies are spread across the CONTRIBUTING.md, often without a version specified (e.g. for mypy or pytest)

I'm happy to help with a PR

Suggest a potential alternative/fix

see above

Start with CONTRIBUTING.md and docs/requirements.txt, then compare the documented setup with the docs build requirements. Clarify the Python and Sphinx constraints and decide how the development dependencies should be represented; done means a new contributor can follow the documented setup without the imghdr failure.