Switch to carpentries/sandpaper instead of our own fork

[milanmlft]

Following carpentries/sandpaper#585, we won't need our own fork of {sandpaper} anymore and I am planning to remove it. This lesson is using that fork for its builds so a small change to the config is required:

• Remove the following line from the config.yaml file:

  aaic2024-neuroimaging-workshop/config.yaml

  Line 85 in 1db0f25

  sandpaper: HealthBioscienceIDEAS/sandpaper@IDEAS

The only reason for having the HealthBioScienceIDEAS/sandpaper fork was to be able to add custom logos to the website, but that's now supported in the upstream carpentries/sandpaper.