(a11y): Add img alt tags / aria-hidden

# Description

Most of our images currently have "image" as alt text. This text is relevant especially for screen reader users. We should give those users the same additional context and information like others.

## Todo

- check if it's possible to add `aria-hidden` to images via markdown - purely decorative images don't need an alt text for most screen readres
- Important: Not all screen reader support aria-hidden, so the images still need an alt text
- add valuable descriptions to images