Divide Phabricator documentation into four books

Summary:
Ref T988. Divides documentation into four books:

  - User: Install, configure and use Phabricator.
  - Contrib: Develop and contribute to Phabricator.
  - Flavor: Worldly advice.
  - Generated: Generated technical documentation.

Test Plan: Generated the books and got sensible results. See screenshots.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D8414