Well, JavaDoc is a mix of HTML and custom tag language based, whereas Kotlin moved to Markdown + some similar tags. Because HTML is a pain in the rear to write compared to Markdown and can't express everything you want concisely.

Maybe you mean Rust's documentation is rendered to HTML. In which case, sure, but so what? Good docs can be rendered to non-HTML formats too, typically for IDEs to display quickly without needing to load a full browser.