Update docs form validation example to demonstrate best-practise when submitting a form in an invalid state

[joshfarrant]

Summary

Primer best-practise dictates that forms submitted with invalid values should automatically move focus to the first invalid input on submit. This PR highlights that fact in the docs and updates our Validation example to demo this. It also removes live validation that runs on change and replaces it with validation that runs on submit.

What should reviewers focus on?

• Check you're happy with the wording, as well as the example

Steps to test:

1. View the Validation section of the Form control page in the docs.
2. Submit the form with an invalid value.
3. Check that the invalid input receives focus.

Supporting resources (related issues, external links, etc):

• Closes https://github.com/github/primer/issues/3760

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Before	After

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Latest commit: 704b30a

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⚠️ Visual differences found

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