feat: Add Google Gemini API support for chat and embeddings (#57)

- Add GeminiChat class with gemini-2.5-flash model support
- Add compute_embeddings_gemini function with text-embedding-004 model
- Update get_llm factory to support "gemini" type
- Update API documentation to include gemini embedding mode
- Support temperature, max_tokens, top_p parameters for Gemini chat
- Support batch embedding processing with progress bars
- Add proper error handling and API key validation

packages/leann-core/src/leann/api.py

@@ -46,6 +46,7 @@ def compute_embeddings(
             - "sentence-transformers": Use sentence-transformers library (default)
             - "mlx": Use MLX backend for Apple Silicon
             - "openai": Use OpenAI embedding API
+            - "gemini": Use Google Gemini embedding API
         use_server: Whether to use embedding server (True for search, False for build)
 
     Returns: