Clean-up old images, compress wolf-sheep image, remove version switch artifact (#2717)

This PR cleans up the docs folder by:
- Removing old, unused images from `docs/images/tutorial` and `docs/tutorials/files`
- Compress the `docs/images/wolf_sheep.png` from 715 KB to 163 KB (77% reduction)
- Remove version switch artifact

In total this removes a little over 2 MB from our repo. It should also make our distribution significantly smaller (3.2 to 1.2 MB).

Author: EwoutH