a11y: add alt text and check if the existing ones are descriptive enough

We should give a general review to all the images/diagrams and make sure we have understandable alt text (if any).

Review all images and diagrams in the tutorial, checking whether each has alt text and whether existing text is understandable. Start by inventorying the tutorial’s images and diagrams; the work is done when each relevant image has descriptive alt text or a clear reason not to need it.

Written by the indexing model from the issue text.