Generate PHP function documentation in Diviner

Summary:
Ref T988. Various improvements:

  - Generate function documentation, mostly correctly.
  - Raise some warnings about bad documentation.
  - Allow `.book` files to exclude paths from generation.
  - Add a book for technical docs.
  - Exclude "ghosts" from common queries (atoms which used to exist, but no longer do, but which we want to keep the PHIDs around for in case they come back later).

This is a bit rough still, but puts us much closer to being able to get rid of the old Diviner.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D6812