feat: adds structured output docs

[theagenticguy]

Description

This pull request introduces a new feature, "Structured Output," to the documentation, providing a detailed explanation of how to use Pydantic models for type-safe, validated responses from language models. Additionally, it updates the API reference to include this feature. The most important changes are grouped below by theme.

Ref: strands-agents/sdk-python#60

Documentation Updates

• API Reference Update: Added strands.agent.agent.structured_output to the API reference in docs/api-reference/agent.md, ensuring the new feature is documented alongside existing functionalities.

• Structured Output Guide: Added a comprehensive guide in docs/user-guide/concepts/agents/structured-output.md explaining the "Structured Output" feature, including its benefits, how it works, usage patterns, advanced features, and best practices. The guide includes code examples for defining Pydantic models, integrating with agents, handling errors, and leveraging structured output in workflows.

Type of Change

• New content addition

Motivation and Context

Reflects the addition of structured output

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.