Subissue: Add documentation for chat commands

[dlqqq]

Problem

There needs to be user & developer documentation for chat commands.

Proposed Solution

• Add developer documentation that explains the chat command registry and describes how to define a custom chat command provider.
• Add user documentation that shows the command menu & explains how the system works

Additional context

We should only do this once the chat commands UI & API are more stable, to avoid re-writing the docs on every change.