Draft: Add section for configuring registry database

What does this MR do and why?

Introduces documentation on how to setup registry experimental metadata database : container-registry#1071 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

• configure gdk with gitlab-docs
• Make sure the branch used by gdk to fetch the gitlab repo is document-registry-database-config and not master
• Start gdk (and make sure gitlab-docs is up and running)
• Navigate to your local gitlab docs page: http://localhost:3005/ee/administration/packages/container_registry.html?tab=Linux+package+%28Omnibus%29#configure-a-metadata-database-for-the-container-registry

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.