Doc: Updated url variable in qdocconf files.

In 5.2, the HTML output is in a flatter structure and when they are
hosted in qt-project.org/doc, the documentation will be found at
http://qt-project.org/doc/qt-$QT_VER

The url variable is used by projects outside of Qt 5 which need
to link to Qt 5 documentation, such as Qt Creator.

Task-number: QTBUG-34584
Change-Id: Ifb9ba5c4a0e78b6170c448ec97f0bec8fa80bdfe
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>