Update docs engine deps

[matejmatuska]

The the previously used version of the docs engine (sphinx) was very old and long unmaintained.

Going with version 7.2.6, which requires Python 3.10, because it's relatively recent (released Sep 13, 2023), also works on Python 3.12 and all the features we need are there. The 3.12 requires manually update of the six package until the version is bumped in requirements.txt.

NOTE: There is still a bunch of warnings, however those don't seem to affect the docs in any way and the documentation is going to be refactored in the near future.

[github-actions[bot]]

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[matejmatuska]

/rerun