Add flatpak integration page

This transfers all the relevant information from https://community.kde.org/Guidelines_and_HOWTOs/Flatpak such that the wiki page can be deleted.

This also adds a lot of information based on my personal digging and searching, and makes it so that a new contributor seeking to package a program as a flatpak can rely on our documentation as a reference too, only looking for upstream's for more complex stuff.

This helped me notice a few gaps in the upstream flatpak documentation to be fixed later.

Closes #102 (closed)

Related: teams/automation/issues#1