Fixes #4270 Add documentation for the new Spring AI - Google GenAI integration modules

[ddobrin]

Changes:
• Auto-configuration fix:

• Added @ConditionalOnClass check for GoogleGenAiEmbeddingConnectionDetails to prevent unwanted instantiation

• MIGRATION_GUIDE.md:

• Added migration guidance (24 lines)

• README updates:

• spring-ai-google-genai-embedding/README.md: Complete rewrite with overview, dependencies, authentication modes
• spring-ai-google-genai/README.md: Updated documentation link from vertexai to google-genai

• Documentation navigation:

• Added Google subsections for both chat and embeddings models
• Organized under "Google" parent items

• Comparison table:

• Added Google GenAI entry with full feature support matrix

New Files Added (2):

• google-genai-chat.adoc

• Complete chat model documentation
• Prerequisites, auto-configuration, properties
• Tool calling, multimodal support, migration guide

• google-genai-embeddings-text.adoc

• Complete embeddings documentation
• Text-only support, task types, dimension reduction
• Migration guide from Vertex AI

Total impact: 8 files

cc: @markpollack @ilayaperumalg @garethjevans