Documentation: Blogpost on gemini thinking config

[priyansh4320]

Why are these changes needed?

Adds a blog post covering AG2 v0.10.3's native support for Google Gemini's Thinking Configuration. The post explains how to use thinking_budget, thinking_level, and include_thoughts to control reasoning depth and transparency. It includes model-specific guidance for Gemini 2.5 and Gemini 3, practical examples for different use cases, advanced patterns for dynamic configuration, best practices, and troubleshooting tips. This helps users understand and leverage Gemini's reasoning capabilities in AG2 agents.

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