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.

Summary by CodeRabbit

• Chores
  • Improved documentation build reliability by standardizing the Node.js environment.
  • Enabled npm dependency caching to speed up future documentation builds.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: f4a505e9-972a-4909-a784-8cd5afc81956

📥 Commits

Reviewing files that changed from the base of the PR and between 1fff30d and 699be37.

📒 Files selected for processing (1)

• .github/workflows/ci.yml

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📝 Walkthrough

Walkthrough

The build-docs CI job now sets up Node.js 20 and enables npm dependency caching using docs-site/package-lock.json.

Changes

Documentation CI

Layer / File(s)	Summary
Configure Node.js and npm caching .github/workflows/ci.yml	The build-docs job adds Node.js 20 setup with npm caching keyed by the documentation lockfile.

Estimated code review effort: 1 (Trivial) | ~2 minutes

✨ 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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