Doc: porting from QDesktopServices::DataLocation to QStandardPaths

Change-Id: Ifd96db46cc8d0170b136a9e3154ed37ebe6ad830
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>