feat(docs): add design principles, globalization, performance, securi…

[yeelam-gordon]

Important to read

With this instruction, when I start GitHub Copilot in this new project:

1. If I ask it to launch the app, it will already know how to build, register, and launch it without issues.
2. If I ask it to add a calendar control where I can add comments, it will add everything needed, fix build errors/warnings, launch the app, and clean up the code.

In contrast, WITHOUT instructions, I ask the exact same questions:

1. It doesn’t know how to build it; since there’s no solution or other instruction, it asks me back.
2. It asks me to add the technology it knows. It does make some sense but I am sure it cannot run to the end...

Summary of this change

This pull request adds comprehensive agent instructions and detailed best-practice guides for WinUI 3 / Windows App SDK C# project templates. The main changes introduce a structured set of markdown instruction files covering design principles, code quality, accessibility, and more. These documents are included in the template package so that agents and developers have clear, authoritative guidance for every aspect of development and review.

Addition of agent instructions and best-practice documentation:

• Added Agents.md, a central guide for agents working on WinUI 3/Windows App SDK projects. It explains how to interpret project configuration, outlines the agent workflow, and provides an index of all instruction files.
• Packaged all instruction markdown files (including Agents.md and those under instructions/) into the VSIX template output, ensuring they are available with the template content.

New instruction documents for best practices:

• Design Principles: Added design-principles.instructions.md, detailing DRY, KISS, SOLID, and YAGNI principles, with rules, anti-patterns, and validation steps.
• Code Quality: Added code-quality.instructions.md, covering static analysis, StyleCop, naming conventions, code cleanup, and validation requirements.
• Accessibility: Added accessibility.instructions.md, specifying accessibility requirements for UI controls, keyboard navigation, screen readers, and validation checklists.

These changes ensure that every code change is guided by clear, enforceable standards for maintainability, accessibility, and code quality, directly within the template package.