fix: validate MDX API anchors and report fragment links in CI

[arunimshukla]

Documentation Pull Request

Summary

Type of Change

• New documentation
• Documentation update/revision
• Fix typos or grammar
• Restructure/reorganize content
• Add examples or tutorials
• Update API documentation
• Other: ___________

Related Issues

Fixes #
Relates to #

Checklist

• Build succeeds locally with pnpm run build
• Lint is successful when running pnpm run check
• Docs follow OpenZeppelin Documentation Standards

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