Diviner is the documentation generator used to build the Phabricator documentation.
You haven't generated any Diviner documentation books yet, so there's nothing to show here. If you'd like to generate your own local copy of the Phabricator documentation and have it appear here, run this command:
phabricator/ $ ./bin/diviner generateRight now, Diviner isn't very useful for generating documentation for projects other than Phabricator. If you're interested in using it in your own projects, leave feedback for us on T4558.