Add Grabcut example

[jbednar]

I think originally https://github.com/pyviz-topics/EarthSim/blob/master/examples/topics/GrabCut.ipynb relied on some custom code for grabbing an ROI on a tile source as an image, but if I recall correctly there is now a good open-source tool for doing that. It would be great to port this example to use such a tool and then replace the EarthSim example with a fully archived one on this website.