Expand C# documentation best practices

[gewarren]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, or chat mode file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, or chat mode with GitHub Copilot.
• I have run node update-readme.js and verified that README.md is up to date.

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Description

Updated the documentation guidelines based on https://github.com/dotnet/dotnet-api-docs/wiki.

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Type of Contribution

• New instruction file.
• New prompt file.
• New chat mode file.
• Other (please specify): Updated prompt file

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[Copilot]

Pull Request Overview

This PR expands the C# documentation best practices by reorganizing and adding comprehensive guidelines from the official .NET documentation standards. The update transforms a basic list of XML comment tags into a structured guide with specific sections for different API types.

Key changes:

• Reorganized content into logical sections (Methods, Constructors, Properties, Exceptions)
• Added detailed guidance for parameter descriptions, return values, and property documentation
• Expanded exception documentation guidelines with specific formatting requirements