If have to go back and find examples, but there's not really much to it at all. A long as it's formatted nicely for reading on a small screen, that's a success.

Most recently I suppose would be the Rust book: https://doc.rust-lang.org/book/

It's a delight on mobile. I've noticed many documentation sites she'd to be based some publishing platform that probably includes a good response mobile design, so it's a feature you can get "for free", yet not every docs site uses such a platform or just hasn't implemented their own responsive behavior.

I'll update if I can remember one I read recently that didn't have a great mobile experience.

It's really a very low bar.