If you’re interested in the general concept of sidenotes and labelling figures in the margin, here’s my take on it for further inspiration: https://chrismorgan.info/blog/2019-website/, with https://chrismorgan.info/blog/rust-fizzbuzz/ exhibiting figures.

Do search around for other sidenote implementations, too. I like the markup and presentation I end up with, but you’ll also find other approaches to mobile support in particular, involving things like checkbox hacks to toggle visibility, or just flat-out requiring JavaScript.