Fix documentation build on ReadTheDocs

[roskakori]

Currently the build on ReadTheDocs fails with the following error:

INFO: pip is looking at multiple versions of sphinx-rtd-theme to determine which version is compatible with other requirements. This could take a while.
ERROR: Cannot install -r docs/requirements.txt (line 2) and Sphinx==7.0.1 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested Sphinx==7.0.1
    sphinx-rtd-theme 1.2.2 depends on sphinx<7 and >=1.6

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/topics/dependency-resolution/#dealing-with-dependency-conflicts

This has already been reported two months ago but not addressed yet: readthedocs/sphinx_rtd_theme#1463. A comment at this issue suggests to switch to the furo theme.

Expected state:

• The CI catches build error before ReadTheDocs. ℹ️ Not sure why it doesn't already. It does build the documentation using make, which if I understand correctly creates its own virtualenv, which should fail because of the dependency conflict above. However, for some reason however it does not. Spank me. CI did not build the docs.
• The documentation on ReadTheDocs builds again. ℹ️ Possibly by switching the theme to furo or by reverting to the Spinx default theme.