The documentation here is written as a set of simple Markdown documents, using GitHubs (or another hosting site) ability to render these nicely and link between the documents. If needs be, this can later be transformed into a full-blown documentation site, with e.g. Sphinx. Given that it is likely that this documentation may end up inside a larger set of documentation, this is probably the easiest format to incorporate this.
The starting point is the index file.