[WIP] Create copilot-instructions.md

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Android framework version

net9.0-android

Affected platform version

.NET 9

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/android/

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

Relevant log output

n/a

Fixes #1161.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.