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. -->

<!-- If resolving an issue, uncomment and update the line below -->

<!-- Closes #[issue-number] -->

## 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/docs/index.mdx

@@ -45,9 +45,9 @@ Llama Stack consists of a server (with multiple pluggable API providers) and Cli
 
 ## Quick Links
 
-- Ready to build? Check out the [Getting Started Guide](https://llama-stack.readthedocs.io/en/latest/getting_started/index.html) to get started.
-- Want to contribute? See the [Contributing Guide](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md).
-- Explore [Example Applications](https://github.com/meta-llama/llama-stack-apps) built with Llama Stack.
+- Ready to build? Check out the [Getting Started Guide](https://llama-stack.github.io/getting_started/quickstart) to get started.
+- Want to contribute? See the [Contributing Guide](https://github.com/llamastack/llama-stack/blob/main/CONTRIBUTING.md).
+- Explore [Example Applications](https://github.com/llamastack/llama-stack-apps) built with Llama Stack.
 
 ## Rich Ecosystem Support
 
@@ -59,13 +59,13 @@ Llama Stack provides adapters for popular providers across all API categories:
 - **Training & Evaluation**: HuggingFace, TorchTune, NVIDIA NEMO
 
 :::info Provider Details
-For complete provider compatibility and setup instructions, see our [Providers Documentation](https://llama-stack.readthedocs.io/en/latest/providers/index.html).
+For complete provider compatibility and setup instructions, see our [Providers Documentation](https://llamastack.github.io/providers/).
 :::
 
 ## Get Started Today
 
 <div style={{display: 'flex', gap: '1rem', flexWrap: 'wrap', margin: '2rem 0'}}>
-  <a href="https://llama-stack.readthedocs.io/en/latest/getting_started/index.html"
+  <a href="https://llama-stack.github.io/getting_started/quickstart"
      style={{
        background: 'var(--ifm-color-primary)',
        color: 'white',
@@ -76,7 +76,7 @@ For complete provider compatibility and setup instructions, see our [Providers D
      }}>
     🚀 Quick Start Guide
   </a>
-  <a href="https://github.com/meta-llama/llama-stack-apps"
+  <a href="https://github.com/llamastack/llama-stack-apps"
      style={{
        border: '2px solid var(--ifm-color-primary)',
        color: 'var(--ifm-color-primary)',
@@ -87,7 +87,7 @@ For complete provider compatibility and setup instructions, see our [Providers D
      }}>
     📚 Example Apps
   </a>
-  <a href="https://github.com/meta-llama/llama-stack"
+  <a href="https://github.com/llamastack/llama-stack"
      style={{
        border: '2px solid #666',
        color: '#666',