[Doc] [1/N] Initial guide for merged multi-modal processor (#11925)

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

docs/source/contributing/model/registration.md

@@ -48,7 +48,7 @@ ModelRegistry.register_model("YourModelForCausalLM", "your_code:YourModelForCaus
 
 ```{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](#enabling-multimodal-inputs).
+Read more about that [here](#supports-multimodal).
 ```
 
 ```{note}