Updated OpenAI configuration and documentation.

- Added OpenAI alike example in .env.example
- Updated README for OpenAI alike server
- Modified API key handling in lightrag_server.py

lightrag/api/lightrag_server.py

@@ -616,7 +616,7 @@ def create_app(args):
             system_prompt=system_prompt,
             history_messages=history_messages,
             base_url=args.llm_binding_host,
-            api_key=os.getenv("OPENAI_API_KEY"),
+            api_key=args.llm_binding_api_key,
             **kwargs,
         )