Add section on deploying flatpak apps, for sysadmins

[federicomenaquintero]

While automating my configuration with saltstack (but it could be ansible or anything else), I have asked myself the following questions:

• Where do flatpak apps store their configuration?
• If the user runs flatseal, where are those changes stored?

I'd like to have a little chapter in the docs that teaches people how to deploy the configuration and overrides for flatpak apps. I'll provide a PR with a starting point.