Matter Application Cluster Logic Document Addition

[silabs-danielV]

This PR adds the Matter Application Cluster Logic document. Its main purpose is to show users how to customize existing applications and become familiar with the Matter application framework.

This document guides the user through creating a Doorlock application and implementing the OnOff cluster in the ZAP GUI. It then explains what happens following the cluster addition, such as what auto-generated code is added to the application. Lastly, example code is given to users for how to implement custom code to modify the OnOff attribute.

[silabs-dimayorg]

I think this document is very well written and provides all the necessary context and details for anyone to follow the steps and understand the process we are trying to show. I added to comments to try to add on that.
Thank you Daniel.

[silabs-dimayorg]

great content!

[selissia]

index.md should be updated to add a link to this file. It is missing other links as well, would be good to add those too