Add a page for errors in union declarations

[BillWagner]

Fixes #52321

Add a new diagnostic page for errors related to union declarations. Note that one error is in the expression tree file because it's a better fit there.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/expression-tree-restrictions.md	Resolve errors and warnings generated from expressions prohibited in expression trees
docs/csharp/language-reference/compiler-messages/union-declaration-errors.md	"Resolve errors and warnings related to union type declarations"
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

Adds documentation coverage for new C# 15 union-related compiler diagnostics by introducing a dedicated union declaration diagnostics page, wiring it into the C# language reference TOC, and relocating the expression-tree-specific union diagnostic to the existing expression tree restrictions page.

Changes:

• Add union-declaration-errors.md documenting CS9370–CS9375 with guidance and links back to the union feature docs/spec.
• Update expression-tree-restrictions.md to include CS9369 (union conversions in expression trees) in keyword metadata and the diagnostics list.
• Update the C# language reference TOC to link the new union diagnostics page and include CS9369 under expression tree restrictions; remove union diagnostics keywords from the generic “sorry” page.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Removes union-related F1 keywords now covered by dedicated pages.
docs/csharp/language-reference/toc.yml	Adds TOC entry for union declaration diagnostics and indexes CS9369 under expression tree restrictions.
docs/csharp/language-reference/compiler-messages/union-declaration-errors.md	New diagnostics page for union declaration errors (CS9370–CS9375).
docs/csharp/language-reference/compiler-messages/expression-tree-restrictions.md	Adds CS9369 metadata and content to the expression tree restrictions diagnostics page.

[adegeo]

LGTM!