docs(useFormState): Add validation (client & server) example

[bennettdams]

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After seeing a great example from Matt Carroll, I decided to add a slightly different version for client- & server-side validation when using the useFormState hook. In the example, the form action returns different messages for different scenarios:

• A username with less than three characters
• The username "foo", which is reserved
• A username with more than three characters

Here's a screen recording of the three:

https://github.com/reactjs/react.dev/assets/29319414/20e707ae-2122-4316-b3ce-5ae3514f5c76

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