qdoc: WebXML output format: Fix issues with example pages

Since QDoc no longer stores example files as nodes in its tree, the
WebXMLGenerator failed to generate output for those files.

Fix this by generating those pages explicitly as needed, overriding
functions from the Generator base class.

Prevent QDocIndexFiles from writing nested <page> elements when
WebXMLGenerator is in use, as that does not work as expected with
shiboken2.

Fixes: PYSIDE-1088
Change-Id: I01c2af2391726f448271fdb810ffc3da923caca5
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>