[docs] update documentations

[yanxi0830]

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What does this PR do?

• Update documentations

Feature/Issue validation/testing/test plan

• https://llama-stack.readthedocs.io/en/latest/

Sources

Please link relevant resources if necessary.

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes?
• [ ] Did you write any new necessary tests?

Thanks for contributing 🎉!