Add copilot instructions for Microsoft Performance Toolkit SDK

[trgibeau]

This pull request adds a comprehensive Copilot instructions document for the Microsoft Performance Toolkit SDK repository. The new .github/copilot-instructions.md file provides validated build and test steps, describes the project structure, outlines CI/CD workflows, details plugin development patterns, and offers troubleshooting guidelines. This documentation is designed to help both new and experienced contributors quickly understand how to work with the SDK and its plugins.

Documentation and onboarding improvements:

• Added .github/copilot-instructions.md to the repository, providing a detailed overview of the SDK, including its architecture, project structure, and key files.
• Included step-by-step build and test instructions, with explanations of expected warnings and test results, to ensure reliable local and CI/CD validation.
• Documented the CI/CD pipeline configuration and how to replicate PR validation checks locally.
• Explained plugin development patterns, target frameworks, and compatibility guidelines for contributors.
• Listed known issues, expected warnings, and troubleshooting steps to assist developers in resolving common problems.