sdk/.readthedocs.yaml: install dependencies from pyproject.toml

[Frosty2500]

We were still trying to install dependencies from the already deleted requirements.txt, now they are being installed from the pyproject.toml.

[s-heppner]

Could you remove the accidental "Â" from your commit title before I merge this?

You should be able to do this by using git commit --amend on the branch rwth-iat:fix_doc_requirements, modifying the commit message and then git push -f origin fix_doc_requirements.