Add container registry doc

This acts as a scratch pad and to discuss design options for the container registry.

Note: At this point, this is very rough. We've explored an alternative model that we think could make sense. The next step would be to figure out if this is at all a direction we wanted to go into and if the trade-offs are in favor of that approach.

Link: https://db-container-registry.about.gitlab-review.app/handbook/engineering/development/enablement/database/doc/container-registry.html

Relatedly, the diagram files and SQL data can be found here: gitlab-org/container-registry!344 (closed)