Summary: Ref T988. Instead of hard-coding the application landing page, make the Diviner root show books if any have been generated. Otherwise, show a helpful message about how to generate documentation locally.
Test Plan:
{F122723}
{F122724}
Reviewers: chad, btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T988
Differential Revision: https://secure.phabricator.com/D8416
