docs: add base to migration guide

[chenjiahan]

Summary

Add the new base config to migration guide.

Checklist

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

[netlify]

✅ Deploy Preview for rsbuild ready!

Name	Link
🔨 Latest commit	02ee5b8
🔍 Latest deploy log	https://app.netlify.com/sites/rsbuild/deploys/66fcf64d51a9eb0008f7a9a3
😎 Deploy Preview	https://deploy-preview-3626--rsbuild.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 66 (🔴 down 15 from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 92 (no change from production) PWA: 60 (no change from production) View the detailed breakdown and full score reports

---

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