docs: refresh Google Gemini model examples

[KarthiDreamr]

Summary

• replace outdated 2024 Gemini model examples in the Google docs
• point readers to the official Gemini models list because model names change frequently
• remove old gemini-pro / gemini-pro-vision guidance in favor of current multimodal Gemini examples
• added mentions of currently available gemini 2.5, 3, 3.1 and 3.5 series models

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