http://craftinginterpreters.com/a-map-of-the-territory.html
https://www.martigeon.com/textbook-themed-webdesign/
I'm not sure how well this would work for shorter notes though.
I ended up doing pretty much the same thing. It doesn't work so well for footnotes that are supposed to be linked to a particular work though.
Here is an example from my site with both footnotes and sidenotes[0]. I style the sidenotes to be distinct from normal paragraphs, which can look great or weird depending on the content.
[0] https://sheep.horse/2017/10/how_you_are_reading_this_page.ht...