Add documentation for configuration lock managers

Add a page which documents how to configure lock managers,
focusing on use of sanlock with the QEMU/KVM driver

* docs/locking.html.in: Docs about lock managers
* docs/sitemap.html.in: Add lock manager config to
  the deployment section

docs/sitemap.html.in

@@ -72,6 +72,10 @@
                 <a href="firewall.html">Firewall</a>
                 <span>Firewall and network filter configuration</span>
               </li>
+              <li>
+                <a href="locking.html">Disk locking</a>
+                <span>Ensuring exclusive guest access to disks</span>
+              </li>
               <li>
                 <a href="hooks.html">Hooks</a>
                 <span>Hooks for system specific management</span>