Improve docs for using Geo on GDK

What does this Merge Request do?

Simplifies documentation for folks who might be using Geo on GDK, but not necessarily developing against it (and, therefore, don't need all the details).

Also adds a configuration step I found necessary to keep the primary node healthy after repeated updates.

CC @gitlab-org/geo-team just FYI.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.