Major rewrite of the flatpak docs

[orowith2os]

I feel that a complete rewrite of the flatpak documentation is due. It's a mess, doesn't explain how to build some types of apps, and some of the features available aren't shown to the user.

As a flatpak maintainer (vpuppr), I've noticed a few core issues, like no explanations on how to use buildbot, or how to submit to the beta flathub, I had to figure that out myself.

This would obviously be a major undertaking, but one well worth the cost imo. I would gladly help with it, if I can (at least on the plain-text side of things)

[orowith2os]

Going to list things that should be listed in the flatpak docs, which I either couldn't find or were hard to find:

only-arches for specifying different sources to pull from for different architectures
more details on how to use the different build sources
how to use different flatpak builder utilities, like the pip generator

[bbhtt]

Some of these are specific to Flathub and not flatpak and are now covered on docs.flathub.org

no explanations on how to use buildbot

https://docs.flathub.org/docs/for-app-authors/maintenance#buildbot

or how to submit to the beta flathub

https://docs.flathub.org/docs/for-app-authors/maintenance#the-repository

only-arches for specifying different sources to pull from for different architectures

https://docs.flathub.org/docs/for-app-authors/maintenance#limiting-the-set-of-architectures-to-build-on

more details on how to use the different build sources

#101 + #102

like the pip generator

There are seperate issues for other generators.

https://docs.flatpak.org/en/latest/python.html#building-multiple-python-dependencies