documentation: Add documentation for the graphql-dgs-extended-validation module

[Prototype-42]

Add a new Advanced documentation entry that explains how to configure and use the the graphql-dgs-extended-validation module to add your own custom validation rules.

While I was adding graphql-dgs-extended-validation to my project, it took me quite some time to figure out how to configure it correctly since there is still no documentation for the module.
I created some custom validation constraints which I then wanted to add to the SchemaDirectiveWiring. By looking into the pull request Add graphql-dgs-extended-validation module #659 I figured out that I had to inject my custom constraints using the customize(ValidationRules.Builder builder) method of the ValidationRulesBuilderCustomizer. This should be documented with code snippets and explanations.

[ClaudenirFreitas]

There is another repository for documentation. This one.
I suppose we can move this issue. thinking