Add a develop documentation app for all GNOME OS and GNOME SDK

I expect some feedback for this one because I am not totally sure how to deal with it. But this is my proposal, and I would be happy to improve it based on feedback.

So while using GNOME OS, there is one thing that I am really missing. It is access to man and info pages for development purpose. But of course those should probably not be installed by default. So having them as a Flatpak application is a way.

This app covers both the SDK and GNOME OS. I am not sure I want to have 2 different applications depending on whether the documentation is part of the SDK or only GNOME OS.

This app uses yelp as default command. So you can use org.gnome.GnomeOSDoc man:open.2 to open yelp on the man page of open(2).

But it is also possible to run flatpak run --command=man org.gnome.GnomeOSDoc open.2 to use the console man.

The app uses the tools from the SDK. It just copies all the documentation files.