Pin the Sphinx version and update the docs theme

[psss]

Seems that the latest docutils-0.21.2 do not include the rst2man.py command any more which broke package and docs building. Let's pin the sphinx version which is compatible with our theme.

Pull Request Checklist

• implement the feature

[psss]

For reference, the rpm build error was this:

2024-07-16 06:51:16.697 logging.py        INFO   cmd [1] | cp /tmp/tmpibff8i6r/docs/header.txt /tmp/tmpibff8i6r/man.rst
2024-07-16 06:51:16.699 logging.py        INFO   cmd [2] | tail -n+8 docs/overview.rst >> /tmp/tmpibff8i6r/man.rst
2024-07-16 06:51:16.702 logging.py        INFO   cmd [3] | sed '/versionadded::/d' -i /tmp/tmpibff8i6r/man.rst
2024-07-16 06:51:16.705 logging.py        INFO   cmd [4] | rst2man.py /tmp/tmpibff8i6r/man.rst > /tmp/tmpibff8i6r/tmt.1
2024-07-16 06:51:16.707 logging.py        INFO   /bin/sh: line 1: rst2man.py: command not found

And here's one of the jobs.

[martinhoyer]

Could have just used the 2b9d4ea commit, but I'm happy not to be in the danger of being git blamed :D