Fix #453: Document custom form validation

[samdark]

Summary

• add a custom validation section to the forms guide
• show built-in UUID validation and a Callback-based card number checksum example
• link to the validator custom rule guide for reusable rules

Testing

• npm run build
• fresh yiisoft/app checkout: composer install, composer require yiisoft/form-model, php check-form-validation.php

Fixes #453

[Copilot]

Pull request overview

Adds documentation to the Yii3 “Working with forms” guide explaining how to implement custom validation beyond built-in attribute rules, specifically demonstrating UUID validation and a Callback-based custom check for UUID v7.

Changes:

• Added a “Custom validation” section to the forms guide.
• Included examples for validating UUID format with the built-in Uuid rule and adding application-specific validation via Callback.
• Linked to the Yii Validator custom rule guide for reusable validation logic.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.