Portkey AI integration with Gemini

[siddharthsambharia-portkey]

This PR adds documentation for integrating Portkey AI with Gemini's LLMs. Portkey AI offers an AI Gateway and Observability Suite, enabling teams to build reliable and efficient AI applications.

Key Features

• Enhanced observability with 42+ metrics and logs
• Advanced routing features (retries, fallbacks, load balancing)
• Semantic caching for cost optimization
• Guardrails for input/output verification and more

Fixes #320

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[siddharthsambharia-portkey]

@markmcd can you take a look at this?