Add some user/narrative documentation on bin_edges

#645 implemented the concept of `bin_edges` and included it in API docs, but didn't have an example of its usage in the narrative docs. This would be good to do because I think users are unlikely to stumble on this in the API docs without some guidance in the narrative docs built around a semi real-world example.

These docs should highlight the pixel convention assumed that the edges are between the "centers" and that the edge pixel are symmetric.

Review the API documentation and the bin_edges implementation introduced by #645, then locate the narrative documentation section where a semi-real-world example fits. Add an example showing the assumed pixel convention: edges lie between pixel centers and the edge pixels are symmetric.

Written by the indexing model from the issue text.