fix(auth): update model authentication info

[Sita04]

Updates to Authentication and Integration Documentation:

models.md:

• Expanded guidance on authenticating with Google Gemini models, including Google AI Studio and Vertex AI, with detailed setup instructions for API keys, environment variables, and service accounts. [1] [2] [3]
• Added Vertex AI Express Mode and Service Account authentication methods, highlighting their use cases and setup steps for production and automation.

Quickstart Guide Enhancements:

• Added links to the updated authentication guide for easier navigation and emphasized the importance of setting up credentials for LLM services. [1] [2]

Deploy to Agent Engine

• Added the missing cloudpickle dependency to the pip install command.

Navigation Update:

• Renamed the "Models" section in mkdocs.yml to "Models & Authentication" to better reflect the updated content.