docs: refresh contributing guide commands

[pengqima]

Pull Request Checklist

• Did you add new tests and confirm existing tests pass? (pnpm test)
• Did you update relevant docs? (docs are found in the docs folder, and guidelines for updating/adding docs can be found in the contribution guide)
• Do your commits follow the Conventional Commits standard?
• Does your PR title also follow the Conventional Commits standard?
• If you have a breaking change, is it correctly reflected in your commit message? (e.g. feat!: breaking change)
• Did you run lint (pnpm run lint:check) and fix any issues? (pnpm run lint:write)
• Did you follow the contribution guidelines?

Updates the contributing guide so the setup and validation steps match the current v5 workspace.

This refreshes the documented Node requirement, pnpm usage, build/test commands, and docs validation commands. It also removes references to stale paths and scripts that are no longer tracked in the repository.

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Summary

Updates the contributing guide so the setup and validation steps match the current v5 workspace.

This refreshes the documented Node requirement, pnpm usage, build/test commands, and docs validation commands. It also removes references to stale paths and scripts that are no longer tracked in the repository.

Why

The existing guide still referenced older contributor workflow details, including Node 18.16.0, pnpm test, pnpm run docs:dev, site/packages/*, and docs/CONTRIBUTING.md. These no longer match the current main branch configuration and can send new contributors down a dead path.

Testing

• pnpm run lint:check

[pengqima]

@pavelm @Richard-Dang @jakehobbs Hi, Could you please review this PR at your convenience? Thank you very much.