Docs: Clarify that fast refresh is a React feature (#67668)
Closes:
[DOC-3052](https://linear.app/vercel/issue/DOC-3052/clarify-that-fast-refresh-is-a-react-feature)
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Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>
Co-authored-by: Zack Tanner <1939140+ztanner@users.noreply.github.com>
