Update the command for building the documentation

[bouweandela]

Description

The command python setup.py build_sphinx is deprecated, see sphinx-doc/sphinx#10040. It is recommended that we use sphinx-build instead.

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[valeriupredoi]

cheers muchly @bouweandela 🍺 Do you know if this is done differently on an OSX so we can test it with a Github Action too? (in another PR of course, if need be)

[bouweandela]

I would think it's the same, you can find the documentation on sphinx-build here: https://www.sphinx-doc.org/en/master/man/sphinx-build.html