Add multiple header images

The current header of for the main webpage is static (using a [`header:`](https://www.fabriziomusacchio.com/blog/2021-08-11-Minimal_Mistakes_Cheat_Sheet/#header-and-teaser-images) entry).

Possible solutions:
 - github action to rotate the yaml
 - add a [carousel](https://jekyllcodex.org/without-plugin/slider/#)