Doc: remove doc config file from Qt WebEngineWidgets submodule

This helps maintaining the doc dependencies.

All Qt WebEngine module docs are now generated in the
/qtbase/doc/qtwebengine/ folder.

Note that you must run qmake -r for the docs to be generated
correctly after applying this patch.

Add Qt WebEngine C++ Classes page that lists the C++ classes
for the submodules.

Modify snippet and example paths accordingly.

Change-Id: I59431c5f766f30b59654ca4e2219b76c79137225
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>