Documentation, clean up, actions, etc.

[Thoemi09]

This is a big PR with many but mostly trivial changes:

• formatted .clang-tidy
• added doxygen documentation to the C++ code and did some trivial clean up (renaming variables, adding [[nodiscard]], fixing include statements, etc.)
• removed the zip_with function which was untested and not used anywhere else
• split the large itertools.hpp into multiple small headers
• added some tests and did some clean up
• removed all sphinx related files from the doc folder and added doxygen files, removed cpp2py dependency
• updated the ghactions workflow to build the documentation and deploy it

Note that some of the changes are rather subjective like splitting the larger header file and cleaning up the code. We can discuss if this is a good idea or if I should stick as much as possible to the existing code.

[Wentzell]

Thank you @Thoemi09 for this huge improvement!
Everything looks good now.