[Doc] Improve server side validation example

[WiXSL]

Related to #7608

I've tried it on simple project's UserCreate component, and it works as expected

[fzaninotto]

I'm sorry, I don't see the difference between before and after. Can you keep the callback async to show the actual difference?

[WiXSL]

I'm sorry, I don't see the difference between before and after. Can you keep the callback async to show the actual difference?

I'm returning the promise and running side effects on success because of returnPromise: #7608 (comment)

That was the issue reported, the lack of redirection following the example as it was previously proposed

[WiXSL]

Mmm, trying it again, the warnWhenUnsavedChanges problem still remains.
I've reverted to the previous version of the example + Side Effects

[slax57]

Also, the dependency array of the useCallback is missing notify and redirect.

Lastly, I think #7608 is mainly about the warnWhenUnsavedChanges problem.
This PR is a welcome improvement but it should not close #7608 IMO.