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: If867f487719da5cccf1b0edd91f620728885b1fd
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>