Add "Who is using mathbox?" docs

[sritchie]

So far, I know of:

• https://www.math3d.org/
• https://kineticgraphs.org/
• https://github.com/QBobWatson/ila textbook
• https://github.com/christopherChudzicki/mathbox-react
• https://github.com/mentat-collective/mathbox-clj
• visualizations at https://sam.zhang.fyi/
• demos listed here https://groups.google.com/g/mathbox/c/Uvyb5fHaLq4,