What sites worked well for you? Anything in particular about how they worked well (or how other sites have not worked well)?

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.