docs: docs: Documentation updates for 0.5.1

[askmanu]

Added comprehensive documentation for the model_fallbacks feature to docs/module.md. The new section includes:

• Examples of string, dict, and mixed fallback configurations
• Detailed explanation of dict keys (id, vertex_location, vertex_credentials, reasoning, api_key, api_base)
• Use cases for high availability, cost optimization, rate limiting, and multi-region deployments
• Explanation of how the feature integrates with LiteLLM's automatic fallback mechanism

The documentation follows the project's style with clear examples, inline comments, and bullet-point use cases. Issue #16

Issue #16