Automate documentation creation

[stdevel]

Is your feature request related to a problem? Please describe.
Currently manpagesand Sphinx HTML documentation needs to be created manually - it would be great to have this automated. E.g. in a GitHub actions job - see also issues #135 and #77.

Describe the solution you'd like
A GitHub actions job should automatically create the documentation.

Describe alternatives you've considered
Some ideas how to automate this: https://github.com/opsi-org/python-opsi#building-the-documentation
cc @okin

Additional context

[okin]

General idea: use sphinx-autodoc for creating package documentation and only create handwritten documents to explain things like workflows, concepts etc.

For this to work having the code itself documented is a necessity.