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

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

docs/source/contributing/model/index.md

@@ -0,0 +1,26 @@
+(new-model)=
+
+# Adding a New Model
+
+This section provides more information on how to integrate a [HuggingFace Transformers](https://github.com/huggingface/transformers) model into vLLM.
+
+```{toctree}
+:caption: Contents
+:maxdepth: 1
+
+basic
+registration
+multimodal
+```
+
+```{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!
+```