Allow Diviner books to have a "preface" section

Summary: Ref T988. This is primarily intended to let us add the "HEY! THIS ISN'T USER DOCUMENTATION" notices to the arcanist and libphutil technical docs.

Test Plan: Added some prefaces, generated docs, looked at them.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D8410