add plot_contingency_matrix / ContingencyMatrixDisplay

[amueller]

I think it would be great if we could add plotting capability for contingency matrices similar to the ones for confusion matrices.
The main difference is that confusion matrices assume that the two sets of classes are the same, while contingency matrices are between arbitrary sets of categories.
This should be a relatively straight-forward but useful addition.

[yash-clear]

@amueller I want to work on this issue ,I am a beginner

[amueller]

Have you checked out the contributor's guide? Adding a new function is a little bit on the bigger side for a first contribution but should be doable.