Doc: Added the Qt Multimedia backends wiki link

The wiki will be linked to from the Qt Multimedia
index page.

Change-Id: I2b60c203f759494cdf12021659847817007639f1
Task-number: QTBUG-30381
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

doc/global/externalsites/qt-webpages.qdoc

@@ -110,3 +110,7 @@
     \externalpage http://wiki.qt.io/BlackBerry
     \title Qt for BlackBerry
 */
+/*!
+    \externalpage http://wiki.qt.io/Qt_Multimedia_Backends
+    \title Qt Multimedia Backends
+*/