The sphinx-maven plugin is a Maven site plugin that uses Sphinx to generate the main documentation. Sphinx itself was origially created by the Python community for the new Python documentation. It uses a plain text format called reStructured Text which it then compiles into a variety of documentation formats such as HTML, LaTeX (for PDF), epub. reStructured Text is similar to Markdown but - at least via Sphinx - has better support for multi-page documentation.
The sphinx-maven plugin is BSD licensed just as Sphinx itself is.