If you are interested in contributing to the API documentation of the Scala standard library (the documentation generated by Scaladoc), please read Generating Scaladoc and Scaladoc for Library Authors first.
If you’d like to contribute to the actual Scaladoc documentation generation tool itself, then please see the Hacker Set Up Guide which covers the steps and workflow necessary work on the Scaladoc tool.
As of Scala 2.13, the Scaladoc tool is maintained but not actively developed. Major development of Scaladoc will progress as a part of Dotty for Scala 3 in the Scaladoc tool.