wow, thanks so much! That's really exciting :-)
I think one good example is from the "Edward Tufte CSS" website: https://edwardtufte.github.io/tufte-css/ They make liberal use of sidenotes as a kind of alternative to footnotes. That site is also (IMO) a worthwhile one to emulate from a web-based publishing perspective. We also use sidebars in this way on a project that I'm working on ( https://jupyterbook.org/intro) and that many other people have used to build their own online books. It doesn't currently use docutils, but I am hoping to transition...
Allow sidebar directive to have no title