[Docs] Convert rST to MyST (Markdown) (#11145)

Signed-off-by: Rafael Vasquez <rafvasq21@gmail.com>

docs/source/design/input_processing/model_inputs_index.md

@@ -0,0 +1,43 @@
+(input-processing)=
+
+# Input Processing
+
+```{eval-rst}
+.. currentmodule:: vllm.inputs
+```
+
+Each model can override parts of vLLM's [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 [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
+
+```{eval-rst}
+.. autoclass:: vllm.inputs.DecoderOnlyInputs
+    :members:
+    :show-inheritance:
+```
+
+### Registry
+
+```{eval-rst}
+.. autodata:: vllm.inputs.INPUT_REGISTRY
+```
+
+```{eval-rst}
+.. automodule:: vllm.inputs.registry
+    :members:
+    :show-inheritance:
+```