Doc: Fix issues in Qt WebKit documentation config file

This change addresses following issues:
    - Replace hard-coded version strings with QT_VERSION et al.
    - Use the word 'Reference' in description for consistency
    - Add a url variable
    - Add a filter for Qt WebKit (usable in Qt Creator Help)
    - Use the correct selector for listing QML types

Task-number: QTBUG-34966
Change-Id: Ic8f404720b02e022da964eadd50cbd140482809c
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>