Provide separate steps for external Geo tracking databases

What does this MR do?

Geo needs two databases on the secondary, and supports either Omnibus PostgreSQL or 3rd party databases.

Geo works with GitLab deployed with helm, CNG.

But, we've only documented how to do helm deployed Geo with Omnibus PostgreSQL.

To provide guidance on CNG with 3rd party databases - which I'm doing in gitlab-org/charts/gitlab!3065 - generic external DB steps need to be teased apart from the from the Omnibus specific Rails steps.

Requested in https://gitlab.zendesk.com/agent/tickets/382516 and actually, I think this would be a pretty common request. If customers are running CNG in cloud, they can get Kubernetes, Redis, and PostgreSQL from their cloud provider. Using Geo to provide DR in another region means they need these steps.

Related issues

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