Fix package building with Sphinx >= 7.0.0

[ycedres]

Sphinx has dropped the integration with setuptools in version 7.0.0:
https://www.sphinx-doc.org/en/master/changes.html#release-7-0-0-released-apr-29-2023

This removal is currently causing issues when building Cobbler package, as the setup.py script is making use of removed code:

[   28s] + /usr/bin/python3 setup.py build '--executable=/usr/bin/python3 -s'
[   28s] Traceback (most recent call last):
[   28s]   File "/home/abuild/rpmbuild/BUILD/cobbler-3.3.3.0+git.5c498dbf/setup.py", line 17, in <module>
[   28s]     from sphinx.setup_command import BuildDoc
[   28s] ModuleNotFoundError: No module named 'sphinx.setup_command'
[   28s] error: Bad exit status from /var/tmp/rpm-tmp.mAzLcE (%build)

This PR simply makes documentation and man pages to be built outside of setup.py script. It's also useful to be able to use the testing container for openSUSE/cobbler.

Backport of cobbler#3434

[m-czernek]

+1, great catch! @ycedres can you also create PRs for all the other branches we need? I.e.:

• uyuni/head
• suma/head
• suma/4.3