Language reference and fundamentals for file based programs

[BillWagner]

Fixes #46498

• Publish the C# speclet for ignored preprocessor directives, and make the updates for the ignored directives used for file based programs.
• Update the articles in the fundamentals and get started sections so they describe file-based programs.

Note: The snippets5000 failure is because our CI build system looks for a csproj file. Without one, the CI build fails. However, these programs don't need a csproj file. However, they must be contained in one single .cs file.

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Internal previews

📄 File	🔗 Preview link
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/language-reference/preprocessor-directives.md	C# preprocessor directives
docs/csharp/specification/toc.yml	docs/csharp/specification/toc
docs/csharp/tour-of-csharp/overview.md	A tour of the C# language

[BillWagner]

@adegeo This is the first PR that will use file-based-programs, those that are built without a .csproj file.

[BillWagner]

Closed and reopened to verify @adegeo 's fix for single file programs.

@gewarren this is ready for review.