Add PyHEP ecosystem shell diagrams to project about page

[matthewfeickert]

Are you going to add info on these files somewhere on the site? The Scikit-HEP shell figure might actually be really nice somewhere at https://scikit-hep.org/about with some bit of text.

Originally posted by @eduardo-rodrigues in #282 (comment)

This is a great idea that @eduardo-rodrigues had that I was reminded of by a comment from @pelmer. 👍

[alexander-held]

This might also be a good fit for the entry page. Currently that lists lots of packages, but for a first-time reader I imagine having some structure like this might be very helpful (even if not all packages are going to be included in the figure).