Add file-based apps reference

[meaghanlewis]

Summary

This pull request adds comprehensive documentation for file-based C# apps in the .NET SDK, targeting .NET 10 and later. It explains how developers can create, build, and run single-file C# applications without a traditional project file, including supported directives, CLI usage, publishing, caching, and best practices for folder layout. Additionally, the new documentation is integrated into the navigation sidebar for easy access.

Documentation addition:

• Added a new file, docs/core/sdk/file-based-apps.md, detailing the concept, usage, supported directives, CLI commands, publishing options, caching behavior, and folder layout recommendations for file-based C# apps in .NET 10+. This includes examples and guidance for native AOT publishing, user secrets, shell execution, and handling implicit build files.

Navigation update:

• Updated docs/navigate/tools-diagnostics/toc.yml to include a link to the new "File-based apps" documentation in the SDK section, making the topic discoverable from the sidebar.

Fixes #49957

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Internal previews

📄 File	🔗 Preview link
docs/core/sdk/file-based-apps.md	File-based apps
docs/navigate/tools-diagnostics/toc.yml	docs/navigate/tools-diagnostics/toc