docs: Add additional indentation to level 3 menu items

formatnetwork.html has a menu item at level 3. libvirt.css
doesn't have a explicit rule for level 3 and level 3 and
level 2 items end up at the same indentation level.

Add an additional 1em indentation to level 3 menu items.

docs/libvirt.css

@@ -106,6 +106,11 @@ h2, h3, h4, h5, h6 {
     border-left: 8px solid #c5dbd8;
 }
 
+#menu ul.l3 li .inactive,
+#menu ul.l3 li .active {
+    padding-left: 3em;
+}
+
 
 #headerLogo {
     position: absolute;