Fix docs MDX parsing and add PR build check

[pratyush618]

Summary

• Fix 3 bare </> characters in docs that break Docusaurus MDX parsing
• Add docs-check.yml workflow that runs on PRs touching docs/** — builds the site without deploying, catches MDX errors before merge

Changes

• mutation-testing.md: >80% → 80%+, <50% → Under 50%
• red-teaming.md: >100k → 100k+
• New .github/workflows/docs-check.yml: triggers on PR, runs npm run build

Test plan

• Scanned all docs/docs/**/*.md files — 0 bare </> outside code fences
• Docs build passes in CI