[Housekeeping] Add Co-Pilot Prompts

[GeorgeLeithead]

Description of Change

This PR adds natural language instructions for CoPilot to assist contributors with its automated code reviews and code completion suggestions and guidance.

More the the docs:

About coding guidelines

You can customize Copilot code review with custom coding guidelines written in natural language. For more information on Copilot code review, see Using GitHub Copilot code review.
With coding guidelines, Copilot can give feedback based on your organization's specific coding style and best practices.
Because Copilot code review is powered by a large language model, it can help with enforcing coding guidelines that are not covered by your linter or static analysis tool.
Coding guidelines are configured at the repository level. You can create and enable up to 6 coding guidelines per repository.

Linked Issues

• Add copilot-instructions.md to get GitHub Copilot up to speed with the toolkit's coding standards and preferences #2543 (comment)

PR Checklist

• Rebased on top of main at time of PR

[pictos]

It looks good, just need to fix some invalid chars and I've a couple of suggestions. I'll mark as Request Changes because of the invalid chars.

[GeorgeLeithead]

It looks good, just need to fix some invalid chars and I've a couple of suggestions. I'll mark as Request Changes because of the invalid chars.

Thanks Pedro. I think I have eliminated all of the invalid characters and also implemented all of the changes you suggested.

[GeorgeLeithead]

@pictos @TheCodeTraveler @bijington @VladislavAntonyuk I had added a new prompt, specifically for MCT, that details some of the common code styles and implementation templates that we use.

Please give them a try out.

I personally appreciate the code reviews provided, but as this is NOT code that the end-user will be provided, please feel free to update this fork/branch as appropriate. The more the better.

[Copilot]

Pull Request Overview

This PR adds several markdown documents that provide natural language guidelines on asynchronous programming, unit testing, memory leak prevention, MAUI controls, and general GitHub Copilot usage for the .NET MAUI Community Toolkit.

• Introduces coding guideline documents for asynchronous operations, unit testing with xUnit, and MAUI-specific practices.
• Provides detailed best practices for memory management, UI controls design, and overall code quality for MAUI projects.
• Adds GitHub Copilot instructions to help guide contributors on using Copilot effectively.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
.github/prompts/dotnet/async.prompt.md	New async programming best practices guideline document
.github/prompts/dotnet/testing.xunit.prompt.md	New unit testing best practices guideline document using xUnit
.github/prompts/dotnet/maui/maui-memory-leaks.prompt.md	New guidelines for handling memory leaks in .NET MAUI applications
.github/prompts/dotnet/maui/maui-controls.prompt.md	New guidelines for page lifecycle and controls in .NET MAUI
.github/copilot-instructions.md	New documentation outlining GitHub Copilot usage and best practices
.github/prompts/prompts.prompt.md	New list of prompt instructions linking all the guideline documents

[pictos]

LGTM, thanks for it @GeorgeLeithead