[Doc] Convert docs to use colon fences (#12471)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/source/contributing/model/registration.md

@@ -17,17 +17,17 @@ After you have implemented your model (see [tutorial](#new-model-basic)), put it
 Then, add your model class to `_VLLM_MODELS` in <gh-file:vllm/model_executor/models/registry.py> so that it is automatically registered upon importing vLLM.
 Finally, update our [list of supported models](#supported-models) to promote your model!
 
-```{important}
+:::{important}
 The list of models in each section should be maintained in alphabetical order.
-```
+:::
 
 ## Out-of-tree models
 
 You can load an external model using a plugin without modifying the vLLM codebase.
 
-```{seealso}
+:::{seealso}
 [vLLM's Plugin System](#plugin-system)
-```
+:::
 
 To register the model, use the following code:
 
@@ -45,11 +45,11 @@ from vllm import ModelRegistry
 ModelRegistry.register_model("YourModelForCausalLM", "your_code:YourModelForCausalLM")
 ```
 
-```{important}
+:::{important}
 If your model is a multimodal model, ensure the model class implements the {class}`~vllm.model_executor.models.interfaces.SupportsMultiModal` interface.
 Read more about that [here](#supports-multimodal).
-```
+:::
 
-```{note}
+:::{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.
-```
+:::