docs: add steering and queueing guide

Add a comprehensive guide covering the two message delivery modes
available through the SDK:

- Steering (immediate mode): inject messages into the current agent
  turn for real-time course correction
- Queueing (enqueue mode): buffer messages for sequential processing
  after the current turn completes

Includes code examples for all four SDK languages (Node.js, Python,
Go, .NET), a sequence diagram, API reference, UI building patterns,
and best practices.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: patniko