OF-1345 OF-1139: 'Mapped' providers should be documented.

Usage of Mapped providers requires a lot of configuration, that should be documented. The documentation provided in this commit describes a common use case, in which admins are to be retrieved from the default providers, while regular users are obtained from an external system.

documentation/docs/index.html

@@ -50,6 +50,10 @@ messaging (IM) services using the XMPP protocol.
         <a href="db-integration-guide.html">Custom Database Integration Guide</a> -
         A guide to integrating Openfire authentication, user, and group data with a custom database.
     </li>
+    <li>
+        <a href="separating-admin-users-guide.html">Separating Administative Users Guide</a> -
+        A guide to setting up Openfire to work with different user stores for admins and non-administrative users.
+    </li>
     <li>
         <a href="pluggable-roster-support-guide.html">Pluggable Roster Support Guide</a> -
         A guide to integrating Openfire rosters with an alternate store.