[MNT] retroactively fix failing readthedocs versions 1.1.0 and 1.1.1

[fkiraly]

Retroactively, the readthedocs versions 1.1.0 and 1.1.1 are failing due to faulty/deprecated .readthedocs.yml settings.

We should try to fix this, although I'm not sure whether it can be done.

A fix would be:

1. replace the .readthedocs.yml at the tag with the current one, and replace the pyproject.toml with one where the current docs depset is present, but make no other changes to pyproject.toml (I am not sure how to do this properly)
2. initiate build on readthedocs of 1.1.0 and 1.1.1