Starting the Calendar user guide

Summary: Ref T7951, Starting the Calendar user guide

Test Plan: Go to Diviner → Phabricator User Docs → Calendar User Guide, read about how fabulous the Calendar application is.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T7951

Differential Revision: https://secure.phabricator.com/D13496