Document metainfo with examples

[Doomsdayrs]

Creating metainfo is sorta "Figure it out" rn.

I had to personally find https://hughsie.github.io/oars/generate.html to generate a content_rating.

[JakobDev]

https://docs.flatpak.org/en/latest/conventions.html?highlight=appstream#appdata-files links already the official specification

[bbhtt]

Flatpak does not impose any restrictions or requirements about MetaInfo. You can build a Flatpak without any MetaInfo file. Those are imposed by software stores or applications.

I don't think duplicating the Appstream spec in Flatpak docs is a good idea. Someone has to update it constantly.

The requirements imposed by software stores should be documented in their docs eg. https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/ and https://docs.elementary.io/develop/appcenter/publishing-requirements#metadata-requirements

(I wrote Flathub's metainfo guidelines page.)