Docs: Mention npm link with Turbopack root (#86075)
## What?
Mention `npm link` case for the Turbopack root for when the package is
outside of the project directory.
Based on feedback here:
https://x.com/stevenaharrison/status/1988781488738070962.
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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
