docs(readme): update available benchmark scenarios and links

[Unisay]

Summary

Update README to reflect current scenarios and improve intra-repo links.

Changes

• Rename “Available Benchmarks” to “Available benchmark scenarios” (and update TOC link)
• Add Factorial scenario row linking to scenarios/factorial.md
• Convert plain USAGE.md mention to a proper Markdown link

Rationale

Keeps documentation accurate as new scenarios are added and ensures links render properly on GitHub Pages and in the repository.

Affected files

• README.md

Verification

• Rendered README locally to confirm anchors and links:
  • scenarios/fibonacci.md
  • scenarios/factorial.md
  • USAGE.md
  • CONTRIBUTING.md
  • LICENSE

Checklist

• Docs updated
• Links verified
• No code changes

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No functional changes; documentation only.