R1:2c870bad8688

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…