examples: Correct API key usage for Groq integration

[Ashad001]

Related To: #890

• Added godotenv to load environment variables from .env file.
• Corrected the initialization of the OpenAI client to include openai.WithToken(apiKey) for proper Groq API key usage.
• Updated the example to reflect the correct usage and included necessary error handling for missing API keys.

This change addresses the confusion regarding the API key parameter and ensures proper integration with the Groq API.

I still think there should be a separate module to load models for Groq instead of OpenAI, for better readability.