Add doclink for Diviner documentation

Summary: Fixes T7502.

Test Plan: Went to /diviner/ and saw a link to the documentation at /help/documentation/PhabricatorDivinerApplication/.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7502

Differential Revision: https://secure.phabricator.com/D12094