Doc: Adding Qt Creator's URL in qt-project.org in qdocconf file.

-can be used for referencing Qt Creator's pages from other
 QDoc projects.

Change-Id: Ifdb82a8995976580876523f5bd6efb688bdfb9b1
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>

doc/config/qtcreator-project.qdocconf

 project     = "QtCreator"
 description = "Qt Creator Manual"
+url         = http://qt-project.org/doc/qtcreator
 
 headerdirs =
 sourcedirs = $SRCDIR/src