Doc: Fix various documentation issues

    - Use 'Qt Canvas 3D' as the module name throughout the
      documentation.
    - Replace unnecessary use of \group with \page commands
    - Fix example group page not being generated correctly
    - Add QML types to the correct module, remove their use
      of \ingroup
    - Replace 'indexes' variable with 'depends' as this is
      no longer a module external to Qt.
    - Fix broken 'exampledirs'
    - Add correct dependencies and ensure linking to other
      Qt module docs works.
    - Style and language fixes
    - Limit line width to 80 characters in .qdoc files

Change-Id: Ib4f4ca59e250db3a5d60c4988701a8fb209438c1
Task-number: QTBUG-45247
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>