Add OpenRouter Integration Examples to models.ipynb Documentation

[avnishs17]

Why are these changes needed?

Adds OpenRouter integration section to the existing models.ipynb notebook. OpenRouter provides access to multiple AI models from different providers (OpenAI, Anthropic, Google, Meta, etc.) through a unified OpenAI-compatible API, but this integration wasn't documented. This addition provides working examples that I've tested locally, demonstrating both basic chat and streaming functionality with OpenRouter models.

Related issue number

N/A - Documentation enhancement

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[avnishs17]

@microsoft-github-policy-service agree