Add registry import metadata database to admin docs

What does this MR do and why?

Related to Create container registry import guides for Omn... (omnibus-gitlab#8255 - closed).

We are documenting the steps to run the registry metadata database import using Omnibus. This MR updates the admin docs with the steps to do so. It uses #423459 (comment 1534203660) as the base for the steps.

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