fix: add cdn output path to turbo config

[ShaunSHamilton]

Summary

 Tasks:    7 successful, 7 total
Cached:    4 cached, 7 total
  Time:    16.398s

    ...Finishing writing to cache...                                                                                                                                                                       WARNING  no output files found for task @freecodecamp/uikit-cdn#build. Please check your `outputs` key in `turbo.json`

Type of change

• feat — new behavior
• fix — bug fix
• refactor — no behavior change
• docs — documentation only
• chore / ci / build — tooling, infra
• test — tests only

Pre-flight checklist

• pnpm changeset added (skip for chore / docs / test / ci-only PRs)
• pnpm test, pnpm typecheck, pnpm lint, pnpm format:check clean locally
  • pnpm test fails for timeouts - local machine too slow
• Visual goldens refreshed if UI changed (pnpm test:visual:update on macOS;
  Linux goldens regenerated via the Playwright Docker image — see
  docs/runbooks/deploy-docs.md)
• apps/docs/public/{_headers,_redirects,robots.txt} unchanged or
  rebaselined deliberately
• ADR added or updated if a locked decision changes (see
  docs/adr/README.md)