Add doc dependencies to setup.py

[mmschlk]

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Add all doc dependencies to setup.py (only top level imports no secondary dependencies). In setup.py line docs_packages is a list of all required packages in typical req.txt format (version numbers etc.) We can update these dependencies to push all changes to pypi and be able to install ixai via pip with all dependencies for creating the docs like so: pip install ixai[docs].