Add governance description for OWNERS and MAINTAINERS files within each repo

[SteveLasker]

We've recently added OWNERS and MAINTAINERS to each repo, attempting to create clarity for who has rights to the repos.
While it would be nice for Github teams to capture this in a declarative format, it's the tools we have today.

This is a tracking issue to define what Github roles are associated with the OWNERS and MAINTAINERS files within the ORAS Governance docs.

As part of the PR, how do folks want to capture Org maintainers? Should that also be capture in the OWNERS and MAINTAINERS file for full transparency? Or, is that captured in org governance?
For reference: notation/MAINTAINERS and notation/CODEOWNERS