docs: add Flatpak documentation

[vially]

This should have been a part of #5711 but, as I've mentioned before, I had some issues getting the documentation system to work on my machine in the past.

After some trial and error I did manage to get it working (fwiw, part of the problem was that version 1.2.4 of mkdocs is not compatible with Python 3.10 but downgrading to Python 3.8 fixed the problem), so now the FlatpakOptions should be properly documented.

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[mmaietta]

Thank you for your contribution!