15.1 docs: React 19 (#73510)
Closes:
- https://linear.app/vercel/issue/DOC-3927/react-19-stable
-
https://linear.app/vercel/issue/DOC-3931/update-mentions-of-useformstate-to-useactionstate-for-react-19-stable
This PR updates the docs for React 19 stable.
- Reviews docs for mentions of React 19 canary
- Updates mentions of `useFormState` to `userActionState`
- Reviews mentions of `useFormStatus`, improves example
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Co-authored-by: Lee Robinson <me@leerob.io>
