chore(ci): enable npm cache in setup-node

[ykstorm]

Summary

Adds a setup-node step with cache: 'npm' to the build-docs job so subsequent runs restore ~/.npm from cache instead of downloading all deps fresh.

Why

npm ci currently runs cold every time; the build-docs job had no setup-node step at all, so no npm cache existed. cache-dependency-path points at docs-site/package-lock.json because npm ci runs in the docs-site/ working directory, not repo root.

Verification

Run the workflow twice; the second run's Setup Node step logs Cache restored from key: ... (cache HIT) in the Actions log. Other jobs (lint/smoke/gitops) are helm/kubeconform-only and untouched.

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/ci-cache-npm-2026-07-18

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