Add castable docs

[brendt]

Docs for laravel/framework#32129

[taylorotwell]

In hindsight it probably would have been nice to pass array $arguments to the Castable object too, so that you from within castUsing you could use an anonymous class...

public static function castUsing(array $arguments)
{
     return new class(...$arguments) implements CastsAttributes {
        get;
        set;
     }
}

I would suggest adding that to 8.x IMO.