Add all diagnostics for async enumerables (C# 8)

[BillWagner]

Fixes #54659

---

Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/foreach-diagnostics.md	Resolve errors and warnings for foreach statements and async enumerables
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

This PR addresses missing documentation for C# 8 async-enumerable-related diagnostics by consolidating foreach/await foreach compiler errors and warnings into a single themed article and wiring up navigation and redirects.

Changes:

• Add a new compiler-messages article covering foreach and async enumerable diagnostics (including CS8412–CS8420 and CS8424–CS8426).
• Update the C# language reference TOC and the “no specifics” fallback page to point relevant F1 keywords to the new article.
• Remove older per-diagnostic pages that are now covered by the consolidated article, and add redirects for the removed paths.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Removes F1 keywords now documented by the new consolidated foreach diagnostics page.
docs/csharp/misc/cs0280.md	Deletes legacy standalone diagnostic page (now covered by consolidated article + redirect).
docs/csharp/misc/cs0279.md	Deletes legacy standalone diagnostic page (now covered by consolidated article + redirect).
docs/csharp/misc/cs0278.md	Deletes legacy standalone diagnostic page (now covered by consolidated article + redirect).
docs/csharp/misc/cs0230.md	Deletes legacy standalone diagnostic page (now covered by consolidated article + redirect).
docs/csharp/misc/cs0202.md	Deletes legacy standalone diagnostic page (now covered by consolidated article + redirect).
docs/csharp/language-reference/toc.yml	Adds the new consolidated topic and removes per-code TOC entries that pointed to deleted pages.
docs/csharp/language-reference/compiler-messages/foreach-diagnostics.md	New consolidated documentation for foreach/await foreach and async enumerable diagnostics.
docs/csharp/language-reference/compiler-messages/cs1640.md	Deletes legacy standalone diagnostic page (now covered by consolidated article + redirect).
docs/csharp/language-reference/compiler-messages/cs1579.md	Deletes legacy standalone diagnostic page (now covered by consolidated article + redirect).
docs/csharp/language-reference/compiler-messages/cs0446.md	Deletes legacy standalone diagnostic page (now covered by consolidated article + redirect).
.openpublishing.redirection.csharp.json	Adds redirects from removed pages to the new consolidated foreach diagnostics article.