Replace script to generates image thumbnails with runtime generation

### Short description

The recent website rewrite added a script that generates image thumbnails, which needs to be run manually. This script should be replaced by a Jekyll (Ruby) plugin which generates those thumbnails when building.

### Location

Site-wide

### Why have this

Not all contributors may be aware of how to run the current script, or have all prerequisites installed locally. A script that does this when building makes sure all works fine when just writing a new blog post and previewing it in Netlify.

### More info

https://github.com/AntennaPod/antennapod.github.io/pull/517#issuecomment-4651381996