feat: add multiagent state & message reset sections

[JackYPCOnline]

Description

This doc updates introduce message & state management in Multiagent premitives.
Graph: 1. Reset on revisit 2. Custom Hook 3. Default Hook 4. Agent level hook
Swarm: 1. Swarm only recive handoff messages so for now use default hook works.

This PR should merge after we merged other multiagent related PRs.

Type of Change

• New content addition
• Content update/revision
• Structure/organization improvement
• Typo/formatting fix
• Bug fix
• Other (please describe):

Motivation and Context

Areas Affected

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
• 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.

[strands-agent]

This repository has been merged into the strands-agents/harness-sdk monorepo and will be archived shortly. All new development happens there.

If this PR is still relevant, please recreate it against the monorepo. The code now lives under site/. Full commit history was preserved, so your base should be findable.

Apologies for the disruption, and thank you for contributing!