QDoc: Path wrong for images placed in /used-in-examples

Image files (assets) get put in a generic used-in-examples directory, while the URL that's generated for the image includes the example name and path to the image as in the code repository. The result is that the link(s) to image(s) in the example give 404s all over the place.

For example, the following example page:

http://doc-snapshots.qt.io/qt5-dev/qtassistant-simpletextviewer-example.html

At the bottom, the link to the file mainwindow.png has the following URL:

http://doc-snapshots.qt.io/qt5-dev/images/used-in-examples/simpletextviewer/documentation/images/mainwindow.png

The image is actually located at:

http://doc-snapshots.qt.io/qt5-dev/images/used-in-examples/mainwindow.png

This is a regression from 5.10, where the images linked correctly:

http://doc-snapshots.qt.io/qt5-5.10/images/used-in-examples/simpletextviewer/documentation/images/mainwindow.png