Practical docs for using Blocking

**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

Potential topics to cover:
* 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:
* Blocking
* What does and doesn't happen
* (future) sticky validation
* Consequence of validation failure
* App-level blocks can also be applied arbitrarily without needing proof of invalid activity, and can be removed
* All blocks are agent-centric; a network-wide block is a consequence of each individual peer blocking the agent

No documentation file, test, or entry point is named. First confirm whether Blocking and Signals have become stable enough to document, then locate the relevant documentation section. Done means adding practical, terse explanations, code examples, and gotchas for the listed signal and blocking topics once the feature is ready.

Written by the indexing model from the issue text.