docs: make local contribution checks optional

[anchenqlw]

Cookbook contribution

Summary

• Content type: Repository infrastructure
• Locale: en-US and zh-CN contributor documentation
• Target audience: Cookbook contributors, including authors without a local Node.js environment
• User problem and expected outcome: Remove local Node.js validation as a submission prerequisite while keeping it recommended for faster feedback. Required GitHub Actions checks and preview review remain mandatory before merge.

Changes

• Mark local validation as optional and recommended in both READMEs.
• Explain in both contribution guides that contributors may open a PR without Node.js and rely on required GitHub Actions.
• Recommend reproducible dependency installation with npm ci --ignore-scripts when running locally.
• Require preview review locally or from the GitHub Actions artifact, instead of requiring every author to run npm run check locally.
• Clarify the same policy in the automated-checks documentation.

Sources and assets

• Public sources used: None.
• Asset owner and license: No assets added or changed.

Specialist review

• Product, code, architecture, security, or regulated-domain reviewer, when applicable: Maintainer review of contribution workflow wording.

Validation

• npm ci --ignore-scripts: PASS
• npm run check: PASS — 49/49 tests, 0 content errors/warnings, links/build/preview successful
• git diff --check: PASS
• English and Chinese guidance reviewed for semantic parity

Author checklist

• Content-specific author/tag/article checks are not applicable to this repository-infrastructure PR.
• No unsupported content, images, credentials, customer data, internal links, or unreleased capabilities were added.
• I ran npm run check and reviewed the documentation diff.
• Every commit contains a Signed-off-by trailer.
• I understand that maintainer Merge to main applies the repository workflow change.