docs: Add multi-agent streaming docs

[mkmeral]

Description

Adds multi-agent streaming docs based on strands-agents/sdk-python#961

strands-agents/sdk-python#608

Type of Change

• New content addition

Motivation and Context

We are adding a new method and event types to multi-agent systems. These require documentation to improve developer experience.

Areas Affected

Streaming and multi-agent

Screenshots

Checklist

• I have read the CONTRIBUTING document
• My changes follow the project's documentation style
• I have tested the documentation locally using mkdocs serve
• Links in the documentation are valid and working
  • Api reference does not work yet technically, as the multiagent.stream_async is not yet published on PyPi
• Images/diagrams are properly sized and formatted
• All new and existing tests pass

Additional Notes

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.