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

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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/notebooks/Alpha_Llama_Stack_Post_Training.ipynb

@@ -14,7 +14,7 @@
         "We will also showcase how to leverage existing Llama stack [inference APIs](https://github.com/meta-llama/llama-stack/blob/main/llama_stack/apis/inference/inference.py) (ollama as provider) to get the new model's output and the [eval APIs](https://github.com/meta-llama/llama-stack/blob/main/llama_stack/apis/eval/eval.py) to help you better measure the new model performance. We hope the flywheel of post-training -> eval -> inference can greatly empower agentic apps development.\n",
         "\n",
         "\n",
-        "- Read more about Llama Stack: https://llamastack.github.io/latest/index.html\n",
+        "- Read more about Llama Stack: https://llamastack.github.io/\n",
         "- Read more about post training APIs definition: https://github.com/meta-llama/llama-stack/blob/main/llama_stack/apis/post_training/post_training.py\n",
         "\n",
         "\n",