Create CQRS Feature Walkthrough Guide

[stkossman]

Create CQRS Feature Walkthrough Guide.

Description:
Create guides/new-feature.md to help onboarding developers understand our Clean Architecture structure.
Provide a step-by-step example of creating a new API endpoint:

1. Defining the Request/Response DTOs.
2. Creating the MediatR Command/Query.
3. Implementing the Validator (FluentValidation).
4. Writing the Handler.
5. Wiring it up in the API Controller.