[Doc] Move CONTRIBUTING to docs site (#9924)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

docs/source/design/input_processing/model_inputs_index.rst

@@ -0,0 +1,39 @@
+.. _input_processing:
+
+Input Processing
+================
+
+.. currentmodule:: vllm.inputs
+
+Each model can override parts of vLLM's :ref:`input processing pipeline <input_processing_pipeline>` via
+:data:`~vllm.inputs.INPUT_REGISTRY` and :data:`~vllm.multimodal.MULTIMODAL_REGISTRY`.
+
+Currently, this mechanism is only utilized in :ref:`multi-modal <multi_modality>` models for preprocessing multi-modal input 
+data in addition to input prompt, but it can be extended to text-only language models when needed.
+
+Guides
+++++++
+
+.. toctree::
+   :maxdepth: 1
+
+   input_processing_pipeline
+
+Module Contents
++++++++++++++++
+
+LLM Engine Inputs
+-----------------
+
+.. autoclass:: vllm.inputs.DecoderOnlyInputs
+    :members:
+    :show-inheritance:
+
+Registry
+--------
+
+.. autodata:: vllm.inputs.INPUT_REGISTRY
+
+.. automodule:: vllm.inputs.registry
+    :members:
+    :show-inheritance: