ci: updates for Python in Debian to follow PEP0668

Because Debian declares its Python install to be EXTERNALLY-MANAGED, following PEP0668, pip will refuse to install packages system-wide. Installation is only possible in virtual environments or separate Python installs.

This can be overriden by passing the --break-system-packages option to pip.

More info at https://peps.python.org/pep-0668/ https://salsa.debian.org/cpython-team/python3/-/blob/db3395e789613c8d01502da8a1be68a976337022/debian/README.venv

In a second commit, add a small note about an outdated information. This is done to trigger a rebuild of the documentation and fix the missing mermaid graph.