Add Qt 3D documentation to 5.5

Despite qtbug44992_qtdoc.diff , building the docs from root still says:

qdoc: warning: "QMake" Cannot locate index file for dependency "qt3d"

Building the docs from the qt3d folder (qtbug44992_qt3d_doclog.txt
) results gives assistant TOC:

Qt3D Core: Empty page with dead links
+ C++ Classes: The page could not be found. 'qthelp://org.qt-project.qt3dcore.550/qt3dcore/'

Qt3D Render (rer) Empty page with dead links
+ C++ Classes: The page could not be found. 'qthelp://org.qt-project.qt3drender.550/qt3drender/'

See http://wiki.qt.io/Checklist-for-Adding-Documentation-for-a-New-Module