Update external posgresql docs to provide instruction on the registry database requirements
Summary
Our docs don't mention that users have to also configure the regsitry database objects
https://docs.gitlab.com/charts/advanced/external-db/
Implementation
The https://docs.gitlab.com/charts/advanced/external-db/ docs instructs the user on how to set up their external database. But all of those instructions currently only consider a gitlab and gitlabhq_production user and database respectively. For now we only mention:
Prerequisites:
An empty database named gitlabhq_production by default.
A user with full database access. See the external database documentation for details.
We should also tell users to provision another user and database for the container registry metadata database.
The Optional. The following items can be further customized if you are not using the defaults section could also be extended with instructions on which keys could be updated to customize the registry user and database names. Currently this seems to be only viable via registry.database.user and registry.database.name. But I believe these will be provided as globals soon, given: