Add common use cases for minimal state app qubes to documentation

[Atrate]

The problem you're addressing (if any)

As mentioned in the original issue (https://github.com/QubesOS/qubes-issues/issues/1006#issuecomment-3182154768):

Solution: The NetVM (in a minimal-state state) would just store network settings, but not anything else. It would be much harder to store a persistent malware that survives reboot, so such multi-stage attack is much less likely to be successful.

That's a great idea, but it's not obvious how to set that up.

The solution you'd like

Make a documentation with common use cases for minimal state app Qubes, e.g. which directories to make persistent for NetworkManager, which to make persistent for common e-mail or communication software (or just ~/.var for flatpak apps), etc.

The value to a user and who that user might be

Anyone who wants to use this feature effectively without having to experiment or dig through lots of documentation of e.g. NM.

Completion criteria checklist

No response

[Atrate]

Example use cases

sys-net

custom-persist.networkmanager   /rw/config/NM-system-connections

Signal qube

custom-persist.homelocalsharekeyrings  /home/user/.local/share/keyrings
custom-persist.homeconfigsignal        /home/user/.config/Signal