[12.x] Add documentation for Laravel’s distinct validation rule with examples

[mohammadRezaei1380]

This pull request introduces a new section to the Laravel validation documentation that explains the distinct validation rule. The distinct rule ensures that all values in an array input are unique, addressing a common validation requirement.

The new section provides a clear explanation along with example code demonstrating how to use the distinct rule with array inputs, helping developers better understand and apply this validation rule effectively.

This addition improves the clarity and completeness of the validation documentation without affecting any existing functionality.

[AhmedAlaa4611]

I think this is documented in https://laravel.com/docs/12.x/validation#rule-distinct