Enable sphinx-design enxtension

[vtoc]

I Like to enable sphinx-design (package available via pip) to allow some more modern design elements. The PR it self doesn't do much, there should be little to no visible impact, but i'd like to have it merged as base for further work starting with a switch to furo-theme.

A good idea where it would get us can be found at the sphinx-design dokumentation:
https://sphinx-design.readthedocs.io/en/furo-theme/

[nigoroll]

I can confirm that after running pip install sphinx_design, make succeeds.
@bsdphk would need to install it on the webserver, too
Once this is in, we probably should start an INSTALL.rst