docs: docusaurus setup (#3541)

# What does this PR do?

- Docusaurus server setup
- Deprecates Sphinx build pipeline
- Deprecates remaining references to Readthedocs
- MDX compile errors and broken links to be addressed in follow-up PRs

<!-- Provide a short summary of what this PR does and why. Link to relevant issues if applicable. -->

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## Test Plan

```
npm install
npm gen-api-docs all
npm run build
```

<!-- Describe the tests you ran to verify your changes with result summaries. *Provide clear instructions so the plan can be easily re-executed.* -->

docs/getting_started.ipynb

@@ -15,7 +15,7 @@
         "\n",
         "[Llama Stack](https://github.com/meta-llama/llama-stack) defines and standardizes the set of core building blocks needed to bring generative AI applications to market. These building blocks are presented in the form of interoperable APIs with a broad set of Service Providers providing their implementations.\n",
         "\n",
-        "Read more about the project here: https://llamastack.github.io/latest/getting_started/index.html\n",
+        "Read more about the project here: https://llamastack.github.io\n",
         "\n",
         "In this guide, we will showcase how you can build LLM-powered agentic applications using Llama Stack.\n",
         "\n",