[docs] add playground ui docs (#592)

# What does this PR do?

- add docs for playground



https://github.com/user-attachments/assets/ddc5edce-eced-4a68-91da-8709005fa531



## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

docs/source/conf.py

@@ -29,6 +29,7 @@ extensions = [
     "sphinx_design",
     "sphinxcontrib.redoc",
     "sphinxcontrib.mermaid",
+    "sphinxcontrib.video",
 ]
 myst_enable_extensions = ["colon_fence"]