Update starcoder2 model card to new standard format

[sanatb187]

What does this PR do?

This PR updates the starcoder2.md model card to follow the new Hugging Face documentation standard, as part of issue #36979.

Changes made:

• Updated section headers and formatting to match the standardized template.
• Cleaned up redundant or unused sections.
• Ensured doc-builder compatibility for consistent deployment.

This change contributes to the ongoing effort to bring all model cards into a consistent, maintainable structure. No functionality or code changes are introduced.

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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 or the forum? Please add a link
  to it if that's the case.
  Discussed in issue: [Community contributions] Model cards #36979
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

Looping in @stevhliu for review, as this PR updates the model documentation to match the current standard.

[sanatb187]

Hi @stevhliu could you please review this PR and lmk if it looks good?

[stevhliu]

Thank you so much for the time and effort you put into this—I really appreciate it. With the number of models in Transformers growing so quickly, I’ve been looking into ways to automate model card generation to keep up with the pace. For now, I’ll be closing this PR, but I’m very grateful for your interest in this project!

Thanks again 🤗