docs: add extension dev and testing docs
Created by: tjkandala
I'm really excited about #18898! I think adding features to the extension API will be easier than ever, but there's still a lot of context in my head, and I want anyone to be able to contribute.
This PR adds MVP development and testing information to the extension background information page. We'll definitely add more to this as we learn what common points of confusion with the new architecture and code organization are.