Update readme docs

[coxipi]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• More documentation, intended especially for readthedocs.

Does this PR introduce a breaking change?

No

Other information:

[Zeitsperre]

I think I know what's wrong here: You're using html_theme_options that are specific to the furo theme. I think we can justify replacing sphinx_rtd_theme with furo (it's nicer, IMO).

[Zeitsperre]

@coxipi

I think we just need to merge the main branch and deal with the strange CSS issue I pointed out. We'll be good to make a release after that!

[coxipi]

Ok, I will push a couple more changes, just checking the docs before

[coveralls]

coverage: 73.991%. remained the same
when pulling 4774e93 on update-readme-docs
into c5bf64f on main.

[Zeitsperre]

@coxipi This is just missing the pygments pin and I think we're good here!