WIP: Describe array order terminology in 2D, 2D+c, 3D, 3D+c arrays

Still working on it, but just thought I'd check this in so that we can tag it as 0.11.
@stefanv I think MRIs might be the most commonly-known 3D images? I don't think we can include one in the repo, as they can be bloaty... Maybe a very low-res version? Or an image we can find online?