This also renames the category of the diagrams post in order to be in
line with the other tutorials.

`fa-lightbulb` -> `fa-user-plus`

- Use image overlays for the cards
- Use mix-blend-mode to make contrast high enough
- Show post image if one is defined

Changed You+HIFIS to HIFIS for YOU (to be discussed)

- Use default font instead of Hermann
- Reduce size by removing the image and some metainformation