Document URL tags

### Problem

We don't currently document or recommend URL tags, but they are important for knowing where to report issues, help with localization, etc.

### Proposal

We should document the url tags we support in AppCenter and probably consider requiring at least the `bugtracker` URL

### Prior Art (Optional)

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-url