External learning resources

[story645]

So got a request to add https://www.python-graph-gallery.com/ to the mpl docs & wondering now if this repo can be used to spawn a second page to replace https://matplotlib.org/stable/resources/index.html#books-chapters-and-articles. or maybe a second version of this repo for that kind of thing, since presumably we want a slightly higher barrier to entry but there are lots of folks building really good learning resources and it might be useful to have a place they can self register.

[jklymak]

That seems a reasonable idea to me. Certainly it's the kind of thing that need not be tied to a specific version.