Update the install docs to include Flatpak info

[mkaply]

Flatpak policies.json goes here:

/var/lib/flatpak/extension/org.mozilla.firefox.systemconfig/x86_64/stable/policies/policies.json

We should make that official in the documentation.

[mkaply]

On Linux (Flatpak), the sandbox cannot see the host's /etc/firefox directory, so system-wide policies are read from the org.mozilla.firefox.systemconfig Flatpak extension instead. Place the file at:

/var/lib/flatpak/extension/org.mozilla.firefox.systemconfig/<arch>/<branch>/policies/policies.json

where <arch> is the Flatpak architecture (for example x86_64 or aarch64) and <branch> is the Firefox branch (typically stable), for example:

/var/lib/flatpak/extension/org.mozilla.firefox.systemconfig/x86_64/stable/policies/policies.json

This is an unmanaged extension, so nothing needs to be installed with flatpak install — just create the directory tree above, drop policies.json in, and Firefox loads it on the next launch. (For a per-user Flatpak install, use ~/.local/share/flatpak/extension/... instead. The same extension can also provide autoconfig preferences under defaults/pref/.)