Improve docs & Add JSON decode example (#121)

examples/quick_start/srt_example_llava.py

@@ -46,6 +46,9 @@ if __name__ == "__main__":
     runtime = sgl.Runtime(model_path="liuhaotian/llava-v1.5-7b",
                           tokenizer_path="llava-hf/llava-1.5-7b-hf")
     sgl.set_default_backend(runtime)
+    # Or you can use API models
+    # sgl.set_default_backend(sgl.OpenAI("gpt-4-vision-preview"))
+    # sgl.set_default_backend(sgl.VertexAI("gemini-pro-vision"))
 
     # Run a single request
     print("\n========== single ==========\n")