forge: Add a prototype CODEOWNERS file

Many forge instances support a CODEOWNERS file as a way of automatically
requesting reviewers.  Forgejo is no different, though the syntax is not
the same as some other forge instances; so put this in the .forgejo
directory.

This version is for experimenting with the format.  I expect to want this
file to be autogenerated from other data at a later stage.

ChangeLog:

	* .forgejo/CODEOWNERS: New file.