Request: Add Documentation for Debugging .NET MAUI Apps Using ProjectReference

### Description

It would be helpful to have official documentation for debugging .NET MAUI framework code from within an external application. There is already a guide provided on how to [pack and use .NET](https://github.com/dotnet/maui/blob/main/docs/DevelopmentTips.md#testing-branch-against-your-project) locally, which is similar. I think it would be worth adding also a guide on using ProjectReference and referencing the projects directly.

### Public API Changes

No

### Intended Use-Case

This would enable contributors and advanced users to:

Step into MAUI source code directly from their application.
Iterate faster without needing to pack and override NuGet caches.
Simplify debugging scenarios where local changes need to be tested in real-world apps.

Start with docs/DevelopmentTips.md and its existing guide on testing a branch against a project. Add a companion guide covering direct ProjectReference use for debugging MAUI framework code from an external application, including the contributor workflow and how to verify that local changes can be tested without packing or overriding NuGet caches.

Written by the indexing model from the issue text.