Should add sample data

It's useful to have a sample dataset to demonstrate affinder. We should provide image pairs for each of the transforms supported by affinder (affine, euclidean/rigid, similarity). See the `sample_data` contribution reference here:

https://napari.org/dev/plugins/contributions.html#contributions-sample-data

Start with the sample_data contribution reference linked in the issue and inspect how affinder exposes its supported affine, euclidean/rigid, and similarity transforms. Add image pairs covering each transform, then verify that the sample data can be used to demonstrate those cases.

Written by the indexing model from the issue text.