Docs: Update note on `@next/third-parties` being experimental (#60372)
At the moment, `@next/third-parties` is only working on canary. This can
be a blocker for those trying it out on `latest`. This updates the
experimental note to recommend installing canary.
The note should be removed once the package is stable.
Closes: https://github.com/vercel/feedback/issues/51035