RPM Support

In #5758, there are Module Content section, stub pages should be create their for each content type group:

     |-- Package
     |-- Advisory
     |-- Modularity
     |-- Distribution Tree
     |-- Comps.xml types
     |-- Custom repository metadata

For each page:
- list and describe supported content types an their uniqueness constraints
- mention any important information about the type (e.g. there can be only one module-defaults per module in a repo version; advisory conflict resolution; distribution tree and sub_repos, etc)
- if there is any workflow specific to a content type, describe it there (e.g when you delete a modulemd from a repo, its RPMs are removed as well; dep solving (in the future), etc)