Add registry key creation documentation with permissions guidance

[Copilot]

This PR adds comprehensive documentation for creating registry keys in C# with important guidance on permissions and security considerations.

Problem Addressed

Users reported that registry code examples only work when running Visual Studio as administrator, and requested documentation about permissions and whether applications can routinely store/update registry values at runtime.

Changes Made

New Documentation Structure

• Created /docs/csharp/programming-guide/file-system/ directory
• Added how-to-create-a-key-in-the-registry.md with comprehensive guidance
• Added section overview page (index.md)
• Updated C# programming guide table of contents

Key Content Added

Permissions Guidance:

• Clear explanation that HKEY_CURRENT_USER doesn't require administrator privileges
• Table showing permission requirements for different registry hives
• Best practices for production applications

Code Examples:

• Basic registry key creation using HKEY_CURRENT_USER (no admin required)
• Robust error handling for access denied scenarios
• Demonstration of permission differences between user and system registry access
• Windows platform attributes for cross-platform compatibility

Security and Best Practices:

• When to use user-specific vs system-wide storage
• How to handle permission failures gracefully
• Security considerations for data validation and storage
• Alternatives to registry for application configuration

Example Usage

The documentation now shows how to create registry keys without requiring administrator privileges:

// This works without administrator privileges
RegistryKey key = Registry.CurrentUser.CreateSubKey("TestApp");
key.SetValue("LastRun", DateTime.Now);
key.Close();

And includes proper error handling for scenarios where permissions are insufficient.

Fixes #30779.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/programming-guide/file-system/how-to-create-a-key-in-the-registry.md	docs/csharp/programming-guide/file-system/how-to-create-a-key-in-the-registry
docs/csharp/programming-guide/file-system/index.md	docs/csharp/programming-guide/file-system/index
docs/csharp/toc.yml	docs/csharp/toc

[BillWagner]

closing this