Migrate docs from Sphinx to MkDocs (#18145)

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

docs/contributing/model/registration.md

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+---
+title: Registering a Model to vLLM
+---
+[](){ #new-model-registration }
+
+vLLM relies on a model registry to determine how to run each model.
+A list of pre-registered architectures can be found [here][supported-models].
+
+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.
+Finally, update our [list of supported models][supported-models] to promote your model!
+
+!!! warning
+    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.
+
+!!! info
+    [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")
+```
+
+!!! 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.