Add Pillow tutorial

After #444 adds Pillow support, I think Pillow would make a nice visual tutorial, like the Matplotlib tutorials. There might be some good inspiration in the [project tutorial], but check the license before copying too directly. I thought I had seen another tutorial that showed how to adjust the colour balance of a photo of Grace Hopper and crop it and so on, but I can't find it now.

Until #444 is complete, use the workaround in that issue.

[project tutorial]: https://pillow.readthedocs.io/en/latest/handbook/tutorial.html