Add documentation on enabling LDAP

What does this Merge Request do and why?

This updates the LDAP documentation now that !2668 (merged) has been merged.

This moves the previous documentation under "Manual setup instructions", adds more details about which user/groups are available, and adds debuggging tips.

Relates to #878 (closed)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.