Doc: Add Sphinx-based Documentation

An update of !942 (closed) with some of the discussion adressed, as I think this would be indeed a great foundation for updated docs.

To note:

• Deployment: @clemenci The ci job to deploy the doxygen docs seem to have moved. If you point me to how this is currently done, I'm happy to add a deployment for this user guide as well.
• Binary files: This MR adds all of the images of the old user guide and some .ppt files under docs/pptx. Maybe it is better to remove those for now?
• File-format: it is also possible to use markdown, which I think is more widespread these days, for sphinx based userguides like this one. Fortunately, thanks to tools like pandoc this is not very consequential and can easily be changed.