chore: add CONTRIBUTING version guardrail check

[Nsanjayboruds]

This PR fixes #7520

It introduces an automated guardrail to prevent outdated dependency version references in contributor documentation.

What changed

• Added .github/scripts/check-contributing-versions.cjs:
  • compares key version references in CONTRIBUTING.md against package.json
  • validates:
    • explicit snippets: "react": "...", "gatsby": "..."
    • prose major references like React <major> and Gatsby (<major>...)
  • exits with a clear, actionable error when mismatches are found
• Added npm script in package.json:
  • check:contributing-versions
• Updated CI workflow .github/workflows/checks.yml:
  • runs npm run check:contributing-versions before build
• Refreshed stale version references in CONTRIBUTING.md to match current dependencies and wording.

Notes for Reviewers

Validation performed:

• npm run check:contributing-versions passes locally.
• make site was previously run successfully in this environment.
• Pre-commit hooks passed and commit is signed.

This change is scoped to docs guardrails and CI checks; no runtime site behavior is modified.

Signed commits

• Yes, I signed my commits.

[l5io]

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