Add file based program tutorial

[BillWagner]

Fixes #46926

Add a tutorial to build a file-based program. Include packages, and define custom types.

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Internal previews

📄 File	🔗 Preview link
docs/core/whats-new/dotnet-10/sdk.md	What's new in the SDK and tooling for .NET 10
docs/csharp/fundamentals/program-structure/index.md	General Structure of a C# Program
docs/csharp/fundamentals/program-structure/main-command-line.md	Main() and command-line arguments
docs/csharp/fundamentals/program-structure/top-level-statements.md	"Top-level statements - programs without Main methods"
docs/csharp/fundamentals/tutorials/file-based-programs.md	Tutorial: Build file-based C# programs
docs/csharp/language-reference/preprocessor-directives.md	C# preprocessor directives
docs/csharp/toc.yml	Taken from https://github.com/dotnet/roslyn/wiki/Samples-and-Walkthroughs
docs/csharp/tour-of-csharp/overview.md	docs/csharp/tour-of-csharp/overview

[adegeo]

I have a lot of feedback on this one 😸 I think it's good feedback but please feel free to push back on whatever you think doesn't make sense. Let me know when you want another review.

[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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[BillWagner]

@adegeo I've addressed all your comments. This is ready for another review.

[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

More information about PoliCheck

Information: PoliCheck | Severity Guidance | Term
For any questions: Try searching the learn.microsoft.com contributor guides or post your question in the Learn support channel.