Move shared example folder to examples/demos/shared

The root examples directory should not contains shared folders. In the
past multimedia was the only module installing things there, now the
qtdoc module started doing the same. Let's clean up both.

Fixes: QTBUG-69784
Change-Id: Id9f98711e1dff3f610bafddc2a8bebbb764e1504
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>