docs: v3 upgrade guide should mention MDX v1 compat options

[slorber]

Motivation

Forgot to mention mdx v1 compat options

Test Plan

preview

Test links

https://deploy-preview-9452--docusaurus-2.netlify.app/docs/migration/v3

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	524a662
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/653a6e6ad095e0000813519e
😎 Deploy Preview	https://deploy-preview-9452--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

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

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 64	🟢 97	🟢 100	🟢 100	🟠 89	Report
/docs/installation	🟠 62	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/category/getting-started	🟠 68	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog	🟠 84	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 56	🟢 97	🟢 100	🟢 100	🟠 89	Report
/blog/tags/release	🟢 90	🟢 100	🟢 100	🟠 80	🟠 89	Report
/blog/tags	🟠 72	🟢 100	🟢 100	🟢 90	🟠 89	Report