Add "Migrating from pre-V24" page to Styling docs.

[rolfsmeds]

The V24 Styling docs are written specifically for how to do styling in V24, but people working on apps that were migrated from earlier versions probably still use /components folder and/or @CssImport / @Stylesheet to load their css.

It would be good to add a page outlining how to continue using the old approaches in V24, and how to upgrade to the V24 system.

---

If you think this issue is important, add a 👍 reaction to help the community and maintainers prioritize this issue.