[Docs] Update Quick Start to Include Examples for Using AG2 with Other Model Providers

[harishmohanraj]

The current Quick Start guide only includes examples for using AG2 with OpenAI. To make it easier for readers who want to integrate AG2 with other model providers like Google Gemini and Azure OpenAI, we should extend the documentation.

Requested Update:

Add code examples for using AG2 with:

• Google Gemini
• Azure OpenAI

Update the Quick Start documentation to:

• Clearly mention supported providers.
• Explain how to configure them.