[Doc] RayServe Single-Host TPU v6e Example with vLLM

[ryanaoleary]

Why are these changes needed?

This PR adds a new guide to the Ray docs that details how to serve an LLM with vLLM and single-host TPUs on GKE. This PR has been tested by running through the steps in the proposed guide and verifying correct output. This PR uses sample code from https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/tree/main/ai-ml/gke-ray/rayserve/llm/tpu.

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[ryanaoleary]

cc: @andrewsykim

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[ryanaoleary]

I went through and updated this guide for the newer TPU Trillium so that it'll be more useful, it no longer requires multi-host since v6e can fit Llama 70B on a single node. I'll create a PR with a separate guide showcasing serving with Llama 405B and multi-host TPUs. cc: @andrewsykim @kevin85421