Documentation for the container registry metadata database

I'm creating an issue to keep track of the documentation. Seems we have a lot of information in lots of places. Let's try to make it easier for users/customers to follow.

Current status of the metadata database

• Omnibus:
  • Enabled in bundled postgres?
    • In progress (first step of many) omnibus-gitlab!8491 (merged)
  • Supported in external db?
    • Yes.
• Charts:
  • Enabled in bundled postgres (non-production)?
    • In progress gitlab-org/charts/gitlab!4342 (merged)
  • Supported in external db?
    • Yes.

Current documentation

Metadata database:

• (omnibus) https://docs.gitlab.com/administration/packages/container_registry_metadata_database/
• (charts) https://docs.gitlab.com/charts/charts/registry/metadata_database/

External database general:

• (main repo) https://docs.gitlab.com/administration/postgresql/external/
• (charts) https://docs.gitlab.com/charts/advanced/external-db/

Merge requests in review

• Omnibus ext db (docs): !196179 (merged)
• Charts ext db (docs): gitlab-org/charts/gitlab!4379 (closed)
• Charts bundled db (code and docs): gitlab-org/charts/gitlab!4342 (merged)
• Omnibus bundled db (code): omnibus-gitlab!8491 (merged)

Other resources

• Nice FAQ and summary
• Multistep Import Video Walkthrough
• Database Quick Start Guide
• Feedback issue (it also includes time estimates to import form the old registry)