Add alt texts to images

QDoc in dev branch now prints out a report for all \image commands that do not include an alternative description for the image. The alt text comes into play if the image could not be loaded, or there is a screen reader in use.

Add alt texts for each \image that is missing it, in curly braces after the image file argument:
https://doc-snapshots.qt.io/qt6-dev/09-qdoc-commands-includingimages.html#image