Doc: Add extensions used by example files to .qdocconf

As the examples include files with extension that are not
included in the global Qt documentation config, add them
to the module's .qdocconf to get the files listed in the
example documentation pages.

Change-Id: Ifc3bf473b67d6acc795ce3f951f0c5269d69b69c
Task-number: QTBUG-40831
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>