CI: Force docs compilation warnings to be raised as errors
📚 Documentation
We frequently have the issue that a part of the code is changed but the documentation gets broken. Since most doc breaking changes only raise warnings, they are often overlooked.
Let's fix all the warnings and force the build to raise future warnings as errors such that we see it in CI.
Possible way to raise errors suggested here on stackoverflow:
make html SPHINXOPTS="-W"