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.