Docs: let's improve "DTO: basic use" page

[sobolevn]

Summary

Right now https://docs.litestar.dev/latest/usage/dto/0-basic-use.html starts with:

and I was like: wait a second!

Here we demonstrate how to declare DTO types to your route handlers. For demonstration purposes, we assume that we are working with a data model User, and already have two DTO types created in our application, UserDTO, and UserReturnDTO.

So, do we demonstrate how to declare them? Or do we have them already declared?

Let's go further: https://docs.litestar.dev/latest/usage/dto/0-basic-use.html#defining-dtos-on-handlers

What does this example show users?

from litestar import post

from .models import User, UserDTO

@post(dto=UserDTO)
def create_user(data: User) -> User:
    return data

When I was reading it - I had no idea what User is, what UserDTO is, why do I need to use dto= at all.

I propose to refactor the first half of this guide, because right now it is really hard to understand.

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