Add extra installations steps for installing Podman in Ubuntu Focal,
since it's not present in the official Ubuntu repos. This is the final
requirement to reinstate Ubuntu Focal support.
Closes#206
Report some Linux versions that were recently supported (Debian 12 /
Fedora 37) in the installation instructions. These instructions where
copied from the Dangerzone wiki, which is why the recently supported
versions were missing.
Copy installation instructions from the Dangerzone wiki [1] into the
Dangerzone source. This has several benefits:
1. Devs can update installation instructions as part of a PR.
2. Users can see installation instructions for previous releases.
The last point is important, because we can update our instructions in
the main branch, without affecting the instructions a user follows from
the website (currently pointing to the Dangerzone Wiki).
Refs #240
[1]: https://github.com/freedomofpress/dangerzone/wiki/Installing-Dangerzone
