Migrate docs from Sphinx to MkDocs (#18145)

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

docs/contributing/model/README.md

@@ -0,0 +1,23 @@
+---
+title: Adding a New Model
+---
+[](){ #new-model }
+
+This section provides more information on how to integrate a [PyTorch](https://pytorch.org/) model into vLLM.
+
+Contents:
+
+- [Basic](basic.md)
+- [Registration](registration.md)
+- [Tests](tests.md)
+- [Multimodal](multimodal.md)
+
+!!! note
+    The complexity of adding a new model depends heavily on the model's architecture.
+    The process is considerably straightforward if the model shares a similar architecture with an existing model in vLLM.
+    However, for models that include new operators (e.g., a new attention mechanism), the process can be a bit more complex.
+
+!!! tip
+    If you are encountering issues while integrating your model into vLLM, feel free to open a [GitHub issue](https://github.com/vllm-project/vllm/issues)
+    or ask on our [developer slack](https://slack.vllm.ai).
+    We will be happy to help you out!