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

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

docs/source/design/multimodal/adding_multimodal_plugin.md

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+(adding-multimodal-plugin)=
+
+# Adding a Multimodal Plugin
+
+This document teaches you how to add a new modality to vLLM.
+
+Each modality in vLLM is represented by a {class}`~vllm.multimodal.MultiModalPlugin` and registered to {data}`~vllm.multimodal.MULTIMODAL_REGISTRY`.
+For vLLM to recognize a new modality type, you have to create a new plugin and then pass it to {meth}`~vllm.multimodal.MultiModalRegistry.register_plugin`.
+
+The remainder of this document details how to define custom {class}`~vllm.multimodal.MultiModalPlugin` s.
+
+```{note}
+This article is a work in progress.
+```
+
+% TODO: Add more instructions on how to add new plugins once embeddings is in.