[Docs] Add useState and useEffect import (#40944)

It makes copy-pasting the code easier since the person will be aware of
the import

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## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Co-authored-by: JJ Kasper <jj@jjsweb.site>

docs/basic-features/data-fetching/client-side.md

@@ -15,6 +15,8 @@ It's important to note that using client-side data fetching can affect the perfo
 The following example shows how you can fetch data on the client side using the useEffect hook.
 
 ```jsx
+import { useState, useEffect } from 'react'
+
 function Profile() {
   const [data, setData] = useState(null)
   const [isLoading, setLoading] = useState(false)