Improve QtConcurrent ImageScaling example to demo new 6.0 features

QtConcurrent got quite a few new API elements in Qt 6.0. An existing QtConcurrent example should be improved to highlight some new elements.

A suggestion of such an existing example could be examples/qtconcurrent/imagescaling. It is a GUI example which scales images. A suggestion for an extension might be to chain further image processing steps together. The example's documentation is currently a screenshot only and should have Qt Concurrent related code snippets too.