Clean up some old cluster-ish documentation

Summary:
Ref T10751. We currently have a placeholder Almanac document, and a fairly-bad-advice section in Daemons.

Pull these into the modern cluster documentation.

Test Plan: 17 phabricator PHDs

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10751

Differential Revision: https://secure.phabricator.com/D15689