Move notebooks to astropy-learn

Do we want to move the notebooks -- not the entire content, *just* the notebooks -- to Astropy Learn? That would make these materials far more discoverable. I think this is a good idea, something we _could_ do, but not necessarily something we _need_ to do.

Start by reviewing the notebooks in this repository and the Astropy Learn destination named in the issue. Clarify whether the move is wanted and identify exactly which notebooks would be transferred; done means only the agreed notebooks are moved and the materials remain discoverable there.

Written by the indexing model from the issue text.