Document how Codeowners works for subgroups

Problem to solve

Many users including GitLab use subgroups to organize members. Users often get confused about how to get subgroups to work with codeowners

Proposal:

Yeah, I think the documentation is actually in a good place overall after taking another look. However, I think the reason we missed this is due to there being no explicit subgroup example and the fact that there's no actual call out that subgroups need to be shared for them to work with code owners.

from #220581 (comment 356461484)

Customer ticket example (internal): https://gitlab.zendesk.com/agent/tickets/159432

Documentation

Update codeowners page: https://docs.gitlab.com/ee/user/project/code_owners.html