text: Title: Docs: DTO tutorial should mention how to exclude from collection of nested union models
  Body: ### Summary
  
  I was going through the documentation on [this](https://docs.litestar.dev/2/tutorials/dto-tutorial/03-nested-collection-exclude.html) it mentions how to exclude the fields from a collection of nested model. However, it needs an update where it mentions how to exclude fields if someone uses a collection of nested union models.
  
  
  For e.g.
  
  ```py
  
  # Assuming correct imports are in place
  
  @dataclass
  class Address:
      street: str
      city: str
      country: str
  
  
  @dataclass
  class Person:
      name: str
      age: int
      email: str
      address: Address
      children: list['Person' | None]
  
  # The DTO will change like:
  
  class ReadDTO(DataclassDTO[Person]):
      config = DTOConfig(exclude={"email", "address.street", "children.0.0.email", "children.0.0.address"})
  
  # We need to provide two zeroes instead of one.
  
  ```
  
  Now there is a line in the document which states 
  
  `Given a generic type, with an arbitrary number of type parameters (e.g., GenericType[Type0, Type1, ..., TypeN]), we use the index of the type parameter to indicate which type the exclusion should refer to. For example, a.0.b, excludes the b field from the first type parameter of a, a.1.b excludes the b field from the second type parameter of a, and so on.`
  
  However, it is not very clear and an example in the document might help.
  
  ### Working without union (as per example)
  <img width="1526" alt="Image" src="https://github.com/user-attachments/assets/a271c46d-af8a-4c60-8a23-bebbfce53665" />
  
  ### Union introduced
  
  <img width="1438" alt="Image" src="https://github.com/user-attachments/assets/36233b8d-839f-40b9-9217-fd7b9fcc1298" />
  
  ### Union introduced (add extra zero to exclude)
  
  <img width="1447" alt="Image" src="https://github.com/user-attachments/assets/0f9b517c-8044-4367-819a-6a66dfb63082" />
  
  
  > [!IMPORTANT]
  > Order of union matters too and therefore key must be changed based on the order.
  
  