The Swagger2Markup project is on GitHub and some more information you'll find by using Google too.
Swagger2Markup converts a Swagger JSON or YAML file into several AsciiDoc or GitHub Flavored Markdown documents which can be combined with hand-written documentation.
AsciiDoc is preferable to Markdown as it has more features. AsciiDoc is a text document format for writing documentation, articles, books, ebooks, slideshows, web pages and blogs. AsciiDoc files can be converted to HTML, PDF and EPUB. AsciiDoc is much better suited for describing public APIs than JavaDoc or Annotations.
You can generate your HTML5, PDF and EPUB documentation via asciidoctorj or even better via the asciidoctor-gradle-plugin or asciidoctor-maven-plugin.
The project requires at least JDK 8.
See also RESTful API Documentation with Swagger and AsciiDoc