[Doc] Update OOT model docs (#18742)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

docs/contributing/model/registration.md

@@ -23,33 +23,32 @@ Finally, update our [list of supported models][supported-models] to promote your
 
 ## Out-of-tree models
 
-You can load an external model using a plugin without modifying the vLLM codebase.
-
-!!! info
-    [vLLM's Plugin System][plugin-system]
+You can load an external model [using a plugin][plugin-system] without modifying the vLLM codebase.
 
 To register the model, use the following code:
 
 ```python
-from vllm import ModelRegistry
-from your_code import YourModelForCausalLM
-ModelRegistry.register_model("YourModelForCausalLM", YourModelForCausalLM)
+# The entrypoint of your plugin
+def register():
+    from vllm import ModelRegistry
+    from your_code import YourModelForCausalLM
+
+    ModelRegistry.register_model("YourModelForCausalLM", YourModelForCausalLM)
 ```
 
 If your model imports modules that initialize CUDA, consider lazy-importing it to avoid errors like `RuntimeError: Cannot re-initialize CUDA in forked subprocess`:
 
 ```python
-from vllm import ModelRegistry
+# The entrypoint of your plugin
+def register():
+    from vllm import ModelRegistry
 
-ModelRegistry.register_model(
+    ModelRegistry.register_model(
         "YourModelForCausalLM",
         "your_code:YourModelForCausalLM"
-)
+    )
 ```
 
 !!! warning
     If your model is a multimodal model, ensure the model class implements the [SupportsMultiModal][vllm.model_executor.models.interfaces.SupportsMultiModal] interface.
     Read more about that [here][supports-multimodal].
-
-!!! note
-    Although you can directly put these code snippets in your script using `vllm.LLM`, the recommended way is to place these snippets in a vLLM plugin. This ensures compatibility with various vLLM features like distributed inference and the API server.

docs/design/plugin_system.md

@@ -30,8 +30,10 @@ def register():
     from vllm import ModelRegistry
 
     if "MyLlava" not in ModelRegistry.get_supported_archs():
-        ModelRegistry.register_model("MyLlava",
-                                        "vllm_add_dummy_model.my_llava:MyLlava")
+        ModelRegistry.register_model(
+            "MyLlava",
+            "vllm_add_dummy_model.my_llava:MyLlava",
+        )
 ```
 
 For more information on adding entry points to your package, please check the [official documentation](https://setuptools.pypa.io/en/latest/userguide/entry_point.html).