Enhance Partial Class Documentation with Real-World Example Separation Across Files.

[shethaadit]

Summary

In this PR, I have

• Clarified partial class usage with examples split across multiple files to reflect real-world scenarios.
• Updated Employee and Coords examples to demonstrate the recommended multi-file structure.
• Added guidance within documentation to prevent confusion and enhance code readability.
• Reinforced best practices for managing large classes, helping developers avoid common pitfalls and potential breaking issues.

Fixes #43217

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Internal previews

📄 File	🔗 Preview link
docs/csharp/programming-guide/classes-and-structs/partial-classes-and-methods.md	docs/csharp/programming-guide/classes-and-structs/partial-classes-and-methods

[BillWagner]

Thanks @shethaadit

I had one formatting suggestion. I'll merge that and then merge this PR after the build finishes.

[BillWagner]

Hi @shethaadit

The CI build failed because there's two Main methods. You'll need to declare one in the csproj file to specify which is meant as the entry point.

[BillWagner]

Thanks @shethaadit

I'll now.