docs: interactive highlighting in code snippets

[yusefnapora]

I was idly checking out docs for webauthn (as one does), and found this really cool presentation of annotated code snippets:

It would be cool to steal this and use it in our docs. Not sure what the simplest implementation would be - ideally it could pull comments out of the source listing (maybe using some special jsdoc tag or something) and spit out the listing with comments removed and a collection of annotation objects with (text, line_number) data we can use to render.