Docs: Add note about RC deps (#68591)
Add helper notes for those struggling to upgrade to React 19 RC when
using Next.js RC.
[NDX-118](https://linear.app/vercel/issue/NDX-118/update-note-on-react-rc-versions-when-upgrading-to-nextjs-15-rc)
---------
Co-authored-by: Amy Burns <amy.burns@vercel.com>
