Example docs: get the example files' HTML page link to the example documentation

This applies to all examples, but for a specific case, look at http://doc-snapshot.qt-project.org/qt5-dev/qtwidgets/desktop-screenshot-main-cpp.html

It would be nice to have a link from that page to the main page for the screenshot example.

In this case, http://doc-snapshot.qt-project.org/qt5-dev/qtwidgets/desktop-screenshot.html

(people can guess the URL, but this is not recommended).