docs: Add offline engine launch example and documentation (#3771)

examples/runtime/engine/launch_engine.py

@@ -0,0 +1,17 @@
+"""
+This example demonstrates how to launch the offline engine.
+"""
+
+import sglang as sgl
+
+
+def main():
+    llm = sgl.Engine(model_path="meta-llama/Meta-Llama-3.1-8B-Instruct")
+    llm.generate("What is the capital of France?")
+    llm.shutdown()
+
+
+# The __main__ condition is necessary here because we use "spawn" to create subprocesses
+# Spawn starts a fresh program every time, if there is no __main__, it will run into infinite loop to keep spawning processes from sgl.Engine
+if __name__ == "__main__":
+    main()