[docs] add openapi spec to docs (#508)

# What does this PR do?
- modify openapi generator to add coming soon tag for unimplemented api
- sphinx-redocs extension for openapi spec to readthedocs page

## Test Plan



https://github.com/user-attachments/assets/b4c7eebc-2361-4198-a987-dbfbcff914cf






## 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/requirements.txt

@@ -7,3 +7,5 @@ sphinx-pdj-theme
 sphinx-copybutton
 sphinx-tabs
 sphinx-design
+sphinxcontrib-openapi
+sphinxcontrib-redoc