Enable building Qt Design Studio example docs as part of the doc builds

The Qt Design Studio examples and their documentation have been moved to 
https://git.qt.io/public-demos/qtdesign-studio/tree/master/examples
 
We need to have a mechanism for pulling the example docs from the examples repository when building the docs or move the docs and the quoted files to the qtcreator repository.
 
It would also be useful to have a link from the Qt Design Studio example doc topics to the git repository, like the Qt examples now do.