Doc: Add a section for Styles

Change-Id: I96af8ea444754e7c3cab954622d0f385b75f9689
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/controls/doc/src/qtquickcontrols-index.qdoc

@@ -82,6 +82,9 @@
       \li \l{Qt Quick Controls Examples}
      \endlist
 
-    \l {Qt Quick Controls Styles QML Types}
+    \section2 Styles
+    \list
+     \li \l {Qt Quick Controls Styles QML Types}
+    \endlist
 
 */