File scoped namespace diagnostics

[BillWagner]

Fixes #51445

Add the undocumented diagnostics related to file-scoped namespace declarations.

While working in this area, pick up the undocumented errors related to global usings and other aliases.

Finally, do a complete edit pass to focus more on resolving the issues than describing them.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/windows-forms/10.0/menuitem-contextmenu.md	Applications referencing both WPF and WinForms must disambiguate MenuItem and ContextMenu types
docs/csharp/language-reference/compiler-messages/using-directive-errors.md	Resolve warnings related to using and declaring namespaces
docs/csharp/language-reference/toc.yml	docs/csharp/language-reference/toc
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Coming in C# 15

[Copilot]

Pull request overview

Consolidates documentation for compiler diagnostics related to using directives, aliases, and file-scoped namespaces by moving previously separate error pages into a single “using directive errors” article and updating navigation/redirects accordingly.

Changes:

• Expanded using-directive-errors.md to include additional diagnostics (file-scoped namespace, extern alias, global using, alias restrictions).
• Removed several older per-error docs/csharp/misc/cs*.md pages and added redirects to the consolidated article.
• Updated TOC and a related WinForms compatibility doc to point at the consolidated content.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Removes error codes that are now documented elsewhere.
docs/csharp/misc/cs2034.md	Removes standalone CS2034 page (now consolidated).
docs/csharp/misc/cs1730.md	Removes standalone CS1730 page (now consolidated).
docs/csharp/misc/cs1681.md	Removes standalone CS1681 page (now consolidated).
docs/csharp/misc/cs1680.md	Removes standalone CS1680 page (now consolidated).
docs/csharp/misc/cs1679.md	Removes standalone CS1679 page (now consolidated).
docs/csharp/misc/cs1671.md	Removes standalone CS1671 page (now consolidated).
docs/csharp/misc/cs0439.md	Removes standalone CS0439 page (now consolidated).
docs/csharp/misc/cs0430.md	Removes standalone CS0430 page (now consolidated).
docs/csharp/language-reference/toc.yml	Updates navigation and display keywords to match consolidation.
docs/csharp/language-reference/compiler-messages/using-directive-errors.md	Adds/rewrites content to cover additional diagnostics and guidance.
docs/csharp/language-reference/compiler-messages/snippets/UsingDirectives/UsingDirective.csproj	Removes snippet project (article no longer references these snippet includes).
docs/csharp/language-reference/compiler-messages/snippets/UsingDirectives/UnsafeExamples.cs	Removes snippet source file (no longer referenced).
docs/csharp/language-reference/compiler-messages/snippets/UsingDirectives/Program.cs	Removes snippet source file (no longer referenced).
docs/csharp/language-reference/compiler-messages/snippets/UsingDirectives/MyClass.cs	Removes snippet source file (no longer referenced).
docs/core/compatibility/windows-forms/10.0/menuitem-contextmenu.md	Updates cross-link to consolidated using/alias guidance.
.openpublishing.redirection.csharp.json	Adds redirects from removed per-error pages to the consolidated article.