docs(feat): added documentation for Fallback model

[mbutler-aws]

Description

Added documentation for fallback model functionality in the Agent class, including configuration examples and usage patterns.

Type of Change

• New content addition

Motivation and Context

Users need clear documentation on how to configure and use fallback models when the primary model fails or is unavailable. This addresses the gap in documentation for this pending feature.

Areas Affected

• API reference documentation for Agent class

Screenshots

N/A

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

This documentation enhancement helps users implement more robust agent configurations with proper error handling and model redundancy.

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