Added content to custom-validators topic (#6457)

* Added content to custom-validators topic

* Update src/data/roadmaps/angular/content/custom-validators@kxRtLsB3y_th8j-HjmJgK.md

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Co-authored-by: dsh <daniel.s.holdsworth@gmail.com>

src/data/roadmaps/angular/content/custom-validators@kxRtLsB3y_th8j-HjmJgK.md

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 # Custom Validators
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+Custom validators in Angular are functions that allow you to define your own validation logic for form controls. They are used when the built-in validators (like `required`, `minLength`, etc.) do not meet your specific validation requirements. A custom validator is a function that returns either `null` if the form control is valid, or an object that represents the validation error if it is invalid. This object typically contains a key-value pair where the key is the error name and the value is a boolean or some details about the error.
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+Learn more from the following resources:
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+-[@official@Defining custom validators](https://v17.angular.io/guide/form-validation#custom-validators)
+-[@video@How to create custom validator in Angular 17](https://youtu.be/3TwmS0Gdg9I?si=1w4EX-HifJ70-CxT)