Building sidenotes into site

I’m presently using Ghost Pro for my site, and I find I like to write with notes – the easiest way to do this in markdown is with footnotes. However, I would like switch to side notes that would appear and disappear as the reader scrolls through. Does anyone have any thoughts on a relatively easy way to adopt this functionality?

Easy is very subjective, but here are some ways I found:

And here is a scholarly comprehensive resource of prior art on the subject:

And in case you need more:

Many thanks for these. Going through them now.