Ensure all images have descriptive alt-text, and captions (where appropriate)

Add alt-text to all images and consider captioning some of them. The caption is now in the
place where the alt-text used to be, and the alt-text is in an extra attribute after the
definition of the image (e.g. [caption text](./path/to/image){alt='Alt text'})

Tasks:
- [ ] list below all figures which we could improve alt-text and captions