- Understand the use case. Who's the audience? What kinds of developers are those - do they like dense documentation, or just want a bunch of code examples so they can move on to more interesting problems that use the API?
- Look for something that can interface with code comments, such as Doxygen for C++. That will encourage engineers to keep documentation up-to-date in line with their existing work.
- Use non-proprietary formats where possible. Markdown + pandoc can be a nice option. Notion looks pretty upfront, but it get's frustrating when the keybindings are slightly non-intuitive and doesn't support how devs want to work. But this kind of depends on how involved the tooling for the existing documentation is. Maybe other stuff is already in one tool, so it makes sense to re-use the existing tool so clients don't have to navigate between tools