Docs: Update offline_engine_api and add links (#3773)

docs/backend/offline_engine_api.ipynb

@@ -18,7 +18,7 @@
     "- Non-streaming asynchronous generation\n",
     "- Streaming asynchronous generation\n",
     "\n",
-    "****To launch the offline engine, `__main__` condition is necessary in your own script because we use \"spawn\" to create subprocesses, for more details please refer to [launch_engine](https://github.com/sgl-project/sglang/blob/main/examples/runtime/engine/launch_engine.py).****\n",
+    "**To launch the offline engine in your python scripts, `__main__` condition is necessary, since we use `spawn` mode to create subprocesses. Please refer to this [simple example](https://github.com/sgl-project/sglang/blob/main/examples/runtime/engine/launch_engine.py) for more details.**\n",
     "\n",
     "Additionally, you can easily build a custom server on top of the SGLang offline engine. A detailed example working in a python script can be found in [custom_server](https://github.com/sgl-project/sglang/blob/main/examples/runtime/engine/custom_server.py)."
    ]