Doc: Fixed links to Qt Multimedia's "Multimedia" overview.

-the links need the [QtMultimedia].

Change-Id: Ie251edaa88995d231248de60f62a8b527d66bba4
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Doc: Added information about WebEngine and WebChannel in "web" topic.

-increases visibility to new modules.
-also bumped the topic page to the front page.

Task-number: QTBUG-40901
Change-Id: I6e9bacbc31c19b885209f51b3485346e88fbbd8d
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>