Create a warning for OpenSUSE and NixOS due to low package availability

It is possible via the third party RPM repository provided for Fedora by Mullvad.

However at this point I think the discussion of whether or not we can install Mullvad VPN and Mullvad Browser has gotten a little off topic from the original question in this thread of if a warning on the recommendations for NixOS or OpenSUSE is something Privacy Guides should consider since some packages may not be available out of the box on those distributions.