Add copilot instructions

[nkolev92]

Basically the nuget docs version of https://github.com/dotnet/docs/blame/main/.github/copilot-instructions.md.

This should allow us to use copilot to write GH docs, or at least do the first version of them.

[Copilot]

Pull Request Overview

This PR adds GitHub Copilot instructions to help AI-assisted documentation writing for the .NET NuGet docs repository. The instructions establish writing guidelines, style conventions, and formatting standards to ensure consistent, high-quality documentation output.

• Establishes comprehensive writing style guide aligned with Microsoft standards
• Defines formatting conventions and file naming rules for consistency
• Includes AI disclosure requirements and workflow guidance

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[learn-build-service-prod]

Learn Build status updates of commit 20c8de9:

✅ Validation status: passed

File	Status	Preview URL	Details
.github/copilot-instructions.md	✅Succeeded

For more details, please refer to the build report.

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 issues. Other issues are also a high priority. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

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