[Doc][2/N] Reorganize Models and Usage sections (#11755)

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

docs/source/contributing/model/registration.md

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+(new-model-registration)=
+
+# Model Registration
+
+vLLM relies on a model registry to determine how to run each model.
+A list of pre-registered architectures can be found on the [Supported Models](#supported-models) page.
+
+If your model is not on this list, you must register it to vLLM.
+This page provides detailed instructions on how to do so.
+
+## Built-in models
+
+To add a model directly to the vLLM library, start by forking our [GitHub repository](https://github.com/vllm-project/vllm) and then [build it from source](#build-from-source).
+This gives you the ability to modify the codebase and test your model.
+
+After you have implemented your model (see [tutorial](#new-model-basic)), put it into the <gh-dir:vllm/model_executor/models> directory.
+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.
+You should also include an example HuggingFace repository for this model in <gh-file:tests/models/registry.py> to run the unit tests.
+Finally, update the [Supported Models](#supported-models) documentation page to promote your model!
+
+```{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}
+[vLLM's Plugin System](#plugin-system)
+```
+
+To register the model, use the following code:
+
+```python
+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
+
+ModelRegistry.register_model("YourModelForCausalLM", "your_code:YourModelForCausalLM")
+```
+
+```{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).
+```
+
+```{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.
+```