Track Velt SuperDoc comments integration docs

[yoen-velt]

Hi SuperDoc team,

Velt maintains a SuperDoc comments integration that adds hosted threaded comments, notifications, and a managed comments sidebar around SuperDoc documents.

I opened a small docs PR here: #3787

The PR adds a short section to the Custom UI comments guide with:

• Velt SuperDoc setup guide: https://docs.velt.dev/async-collaboration/comments/setup/superdoc
• Live demo: https://sample-apps-superdoc-comments-demo.vercel.app
• Demo source: https://github.com/velt-js/sample-apps/tree/main/apps/react/comments/text-editors/superdoc/superdoc-comments-demo

I kept the change scoped to the comments guide rather than the Yjs collaboration provider docs, since Velt's SuperDoc package is focused on hosted comments UI/annotations. Happy to adjust placement or wording if another docs location fits better.

[linear-code]

IT-1244