docs: `useFormState` (#55564)
This PR shows how to use a new React hook `useFormState` in the context
of the [Forms and
Mutations](https://nextjs.org/docs/app/building-your-application/data-fetching/forms-and-mutations)
docs. It also updates the forms example (`next-forms`) to show the
recommended patterns for loading / error states.
Related: https://github.com/vercel/next.js/pull/55399
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Co-authored-by: John Pham <johnphammail@gmail.com>
