Document how to preview changes

What does this Merge Request do?

A common question many GDK users ask is "how do I use GDK to preview changes in GitLab"? This MR attempts to answer that question.

I also took the opportunity to centralize configuration things on the configuration page, and commands things on the commands page, so the index.md could have some weight taken off it.

Closes gitlab#230823 (closed).

CC @deuley @mikelewis @rpaik

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.