Doc: Fix example paths referred in documentation

qdocconf files for both Qt Multimedia and Qt Multimedia Widgets already
define the correct example path. This change fixes also the example
documentation to use the correct path, as well as the link to examples
in Multimedia Overview.

Change-Id: I72d75027ee18f6398eb76dc40bcc3d122541b1c9
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>