Migrate docs from Sphinx to MkDocs (#18145)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/deployment/frameworks/modal.md

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+---
+title: Modal
+---
+[](){ #deployment-modal }
+
+vLLM can be run on cloud GPUs with [Modal](https://modal.com), a serverless computing platform designed for fast auto-scaling.
+
+For details on how to deploy vLLM on Modal, see [this tutorial in the Modal documentation](https://modal.com/docs/examples/vllm_inference).