The most valuable documentation appears to be the one you add to every type and method, since people can access it from their IDE, and you can automatically compile it into HTML, which you can put on a website dedicated to the library. Don’t forget to have a sub folder for each published version.
Besides Scaladoc, you want to have a tutorial with examples, probably also added to the same website. For maximum community involvement, you can have a wiki.
A forum, for example a Google group, helps with community building.
If your library code lives on GitHub, as most do, you want to add a README.md, which is written in Markdown, and explains at least the essentials, like what this is, and how to get started, including links to the website and the forum. The README can be as extensive as you want, and it is very helpful that it is version-controlled together with the code, although it will never be as visually appealing as a decent website.