Add a new "end user documentation" page

[WhyNotHugo]

Current documentation mainly targets packagers and developers. Through
conversations in GitHub, different requirements have come up that aren't
really documented anywhere.

The intention of this new page is to document everything that a user
needs to install flatpak and run flatpak applications.

In reality, this is mostly for technical users setting up their own
environments, or, potentially, distributions packages.

See flatpak/flatpak#2861 for more background.

[WhyNotHugo]

Any feedback on this?

[TheEvilSkeleton]

I don't think we should have developer docs and end user docs in the same place.

[WhyNotHugo]

What do you mean?

[TheEvilSkeleton]

You are suggesting that we should have end user documentation in docs.flatpak.org. However, that website is tailored for developers, not users. We shouldn't change it in the way it will be tailored for both. It will be really difficult to maintain and the reader will have trouble where to look for.

[WhyNotHugo]

The website also includes the command reference, which really targets end-users (its essentially the same contents as a man page): https://docs.flatpak.org/en/latest/flatpak-command-reference.html

Where do you think the documentation in this patch should be included instead?

[TheEvilSkeleton]

I haven't thought about where it should be. I just think that putting end user docs inside developer docs is a unusual.

[WhyNotHugo]

I didn't think of this site as "developer docs", they're just "the docs" as far as I can tell. The existing user documentation is already part of this site (e.g.: reference for the flatpak command), and as you pointed out, there's no other place with any user documentation anyway.

I'm not sure what you're expecting me to do.

[TheEvilSkeleton]

Fair point. My apologies.

[orowith2os]

This may be possible to include in the "using flatpak" page, rather than its own. I can look into making sure everything is still correct, and include if you'd like

[WhyNotHugo]

That might be a better fit and we can avoid the term "end user". I'm fine with the change if that'll move the patch forward.

[orowith2os]

I've merged the portal files, into one file (on my fork), named portals.rst that contains most of the information here and the QT + GTK file contents. I believe this PR can be closed now.