Update copilot-instructions.md

### Description

Go through this repo, review structure of project, source code, etc.

Additional docs to review about the product: https://learn.microsoft.com/en-us/dotnet/ios/

Update `copilot-instructions.md` to make Copilot more helpful going forward.

See: https://docs.github.com/en/copilot/customizing-copilot/adding-repository-custom-instructions-for-github-copilot
