Spring AI Add documentation for the new Spring AI - Google GenAI integration module

2 modules are available to support the Spring AI integration with Gemini through the Google GenAI SDK:

• models/spring-ai-google-genai
• models/spring-ai-google-genai-embedding

Auto-configuration: auto-configuration/spring-ai-autoconfigure-model-google-genai (for both)

Documentation needs to be added for these modules. Please assign this to me