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

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

docs/source/design/input_processing/input_processing_pipeline.rst

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+.. _input_processing_pipeline:
+
+Input Processing Pipeline
+=========================
+
+1. Input data is passed to :class:`~vllm.LLMEngine` (or :class:`~vllm.AsyncLLMEngine`).
+
+2. Tokenize the data if necessary.
+
+3. Process the inputs using :meth:`INPUT_REGISTRY.process_input <vllm.inputs.registry.InputRegistry.process_input>`.
+
+   - For example, add placeholder tokens to reserve KV cache for multi-modal embeddings.
+
+4. Send the processed inputs to :class:`~vllm.executor.executor_base.ExecutorBase`.
+
+5. Distribute the inputs via :class:`~vllm.worker.worker_base.WorkerBase` to :class:`~vllm.worker.model_runner_base.ModelRunnerBase`.
+
+6. If the data contains multi-modal data, convert it into keyword arguments using :meth:`MULTIMODAL_REGISTRY.map_input <vllm.multimodal.MultiModalRegistry.map_input>`.
+
+   - For example, convert a :class:`PIL.Image.Image` input to its pixel values for a vision model.

docs/source/design/input_processing/model_inputs_index.rst

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+.. _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: