Release aftermath

[PicoCentauri]

Some infrastructure updates after the release:

1. Update the CHANGELOG file for the next release cycle.
2. Remove the leftovers from the black and isort linters. I think we moved to ruff already a while ago...
3. Add a step-by-step guide on how to perform a release.
4. Add banners for the stable and the latest (development) docs on the README.rst.
5. Use setuptools-scm for taking care of the skmatter version. The version will be dynamically generated from the last git tag and the last git commit. Since the last git tag was v0.3, the current version displayed from skmatter.__version__ (for this commit) is 0.4.dev0+g6d7a3f8.d20250613. This makes it easier to compare versions during development.
6. Adding a Cite this repo link on the GitHub sidebar of our repo based on a CITATION.cff file. We did this for other repos: