qdoc: images in our documentation have a low resolution

Take a look at https://doc.qt.io/qt-5/qtquickcontrols2-styles.html

The images on this page have a very low resolution. On HighDPI platforms the images should probably have a 2x size that are scaled down by setting an explicit width height (in css or the img tag).

The result is that, in the example link, the styles look worse than they are in reality.