Document how to register cluster devices with Almanac

Summary:
Ref T4292. This is a required step in configuring a cluster: document and explain it.

Previously bin/almanac register could also add and trust keys. I've removed this capability since I think it's needless and complicated. If there's some real use for it eventually, we could add a bin/almanac add-key or whatever. The workflow is simpler and has better guard rails that point you in the correct direction now.

Test Plan:

• Read documentation.
• Ran bin/almanac with various good/bad flags.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4292

Differential Revision: https://secure.phabricator.com/D15795