docs: add getDataByPath to useForm table (#12098)

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Fixes #

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### What?
This PR adds the `getDataByPath` utility function to the `useForm` table
in the docs.

### Why?
This utility is useful for dealing with arrays and other form state.
Users commonly ask for examples on how to deal with arrays when using
`useField` as this function will return a length instead of the expected
value object.

### How?
Addition of `getDataByPath` to the `useForm` object table in
`docs/admin/react-hooks.mdx`.