feat: Add docs about conversation-managers being HookProviders

[zastrowm]

Description

Per strands-agents/sdk-python/pull/1374 update docs with the latest information on being conversation-managers.

I also updated the heading levels to be more consistent; for some reason we jumped from 2 -> 4

Type of Change

• New content addition

Motivation and Context

strands-agents/sdk-python/pull/1374 is updating conversation-managers to be HookProviders; this documents that

Areas Affected

• conversation-management.md

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

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