Doc: Make documentation versioning follow .qmake.conf...
Doc: Make documentation versioning follow .qmake.conf
No reason to have hard-coded version numbers in .qdocconf,
as they are available from env. variables.
Change-Id: I4a118fe0127a20f01af157fbc496e3cb0ba69976
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>