feat: add new C# developer roadmap

[Venkatachla]

Summary

This PR introduces a new C# Developer Roadmap to the project, aligned with the existing roadmap structure and formatting standards.

Added

• /src/data/roadmaps/csharp/csharp.json
• /src/data/roadmaps/csharp/csharp.md
• /src/data/roadmaps/csharp/faqs-csharp.astro
• /src/data/roadmaps/csharp/migration-mapping-csharp.json

Notes

• No existing files were modified or removed
• Follows the current folder-based auto-registration system (no index update required)
• Matches structure and conventions used in the Java roadmap
• All content is self-contained and ready for future UI rendering

Motivation

C# remains one of the most widely-used languages for backend, cloud, desktop, and game development. Adding this roadmap will benefit developers looking for structured guidance within the .NET ecosystem.

Checklist

• Files placed in correct directory
• Naming follows roadmap standards
• No breaking changes introduced
• Content follows neutral and educational tone