Update Gitaly docs for multiple nodes

What does this Merge Request do?

This merge request adds instructions for how to configure multiple Gitaly nodes and virtual storages for Praefect.

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.