Practical guide for using Countersigning

**This is not a stable feature in holochain core and should not be documented yet.**

Must include:
- code examples
- practical, terse explanation of how to *use*
- gotchas

* Signals
* Local signals to UI
* Remote signals to peers on same DHT
* needs cap grant on `recv_remote_signal`
* (future) pub/sub to replace most use cases for remote signals
* not supported between zomes or between cells :(

Possible topics include:
* Countersigning
* Automated process; agreement to countersign a given piece of data should have already been made
* online and interactive (in distributed systems terms)
* Document the heck out of the lifecycle
```mermaid
sequenceDiagram
box Alice
participant ACell as Alice's cell
participant ACond as Alice's conductor
end
box Bob
participant BCond as Bob's conductor
participant BCell as Bob's cell
end

ACell->>ACell: prepare entry to be countersigned
ACell->>BCell: send entry data (signal or zome function)
BCell-->>ACell: acknowledge, store, and approve of entry data
ACell->>ACell: create reflight request
Note right of ACell: contains parties involved, hash of entry to be signed, time window, action stub, arbitrary preflight bytes
ACell->>BCell: call arbitrary zome function called `receive_preflight` with prepared preflight request
BCell->>

```
* Patterns for implementing the interactions that aren't handled at the system level

The issue names no documentation files or tests. First confirm with maintainers whether Countersigning is stable enough to document, then locate the relevant Holochain documentation entry points and examples; done means a practical guide covering lifecycle, signals, cap grants, gotchas, and interaction patterns.

Written by the indexing model from the issue text.