flatpak: Update with reference runtimes and runtime creation

NOTE: This is a draft because the Flatpak runtime repo's CI is not fully functional yet, but the MR itself should be ready.

This adds a document detailing how to create a new Apertis-based Flatpak runtime using apertis-flatdeb.

This updates the current Flatpak distribution document with information on both the new reference runtimes and creating new, custom runtimes.

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com