add nvidia nim inference provider to docs

[mattf]

What does this PR do?

add NVIDIA NIM reference to the docs

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,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.

[dineshyv]

can we add a section here too? https://llama-stack.readthedocs.io/en/latest/distributions/index.html

[mattf]

can we add a section here too? https://llama-stack.readthedocs.io/en/latest/distributions/index.html

@dineshyv can we do that as part of #565, which has changes to add a distribution while this PR focuses on the provider?

[dineshyv]

sounds good.

[mattf]

@dineshyv will you merge this?