The process is quite easy. Their documentation is near the source code in Git, so that they can benefit from Git (tag, merge/conflict).
It is written with Markdown so it is easy to write it with a simple text editor. Then it is converted to HTML files using the Pegdown library.
For an example of this process, you can take a look at this project: https://github.com/ndeverge/play2-docs-renderer
It converts on the fly the Markdown file from the Github repo to HTML files. You can see the result here: http://play2-docs-snapshot.herokuapp.com/