docs: update relay example to use SWC plugin

[chenjiahan]

Summary

Update relay example to use SWC plugin, the rspackExperiments.relay option will be removed in Rspack v1.0.0.

Related Links

web-infra-dev/rspack#6862

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[netlify]

✅ Deploy Preview for rsbuild ready!

Name	Link
🔨 Latest commit	ca2bd63
🔍 Latest deploy log	https://app.netlify.com/sites/rsbuild/deploys/667904facd5025000891cafd
😎 Deploy Preview	https://deploy-preview-2678--rsbuild.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 75 (🔴 down 6 from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 92 (no change from production) PWA: - View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify site configuration.