Doc: Add Qt Quick Layouts to the list of essential modules

Change-Id: I5c06aa2970c289d02e3f40869440f19227eb2f84
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

doc/src/qtmodules.qdoc

@@ -77,6 +77,10 @@
         \li \l{Qt Quick Controls}
         \li Reusable Qt Quick based UI controls to create classic desktop-style
             user interfaces
+    \row
+        \li \l{Qt Quick Layouts}
+        \li Layouts are items that are used to arrange Qt Quick 2 based items in
+            the user interface.
     \row
         \li \l{Qt SQL}
         \li Classes for database integration using SQL.