Add LDAP multi-host configuration instructions

What does this merge request do and why?

Add instructions for setting up a multi-host LDAP configuration to the LDAP HOWTO.

It is not trivial to set up a local environment such that GDK points to multiple functioning LDAP hosts. It will be useful to document this setup for scenarios like reproducing bugs or testing LDAP flows during dependency upgrades (e.g., this net-ldap gem bump motivates the change).

References

• gitlab#570587 (closed)

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.