Add diagnostics for record struct and record class declarations

[BillWagner]

Fixes #51449

Add a new file with the diagnostics for record struct and record class types.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/record-declaration-errors.md	Resolve errors and warnings for record 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

This PR documents C# 9 and C# 10 compiler diagnostics related to record, record struct, and record class declarations, and wires the new documentation into the C# language reference navigation so F1/help keyword routing can land on a specific article instead of the generic “no specifics” page.

Changes:

• Add a new “Record declarations” compiler-messages article covering CS8851, CS8858–CS8866, CS8870–CS8879, CS8907–CS8908, and CS8913.
• Add the new article to the C# language reference TOC under “C# compiler messages”.
• Remove newly documented diagnostic IDs from the generic “Sorry, we don’t have specifics…” fallback list.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Removes record-related diagnostic IDs that are now covered by a dedicated article.
docs/csharp/language-reference/toc.yml	Adds a new TOC entry for the record-declarations diagnostics article.
docs/csharp/language-reference/compiler-messages/record-declaration-errors.md	New documentation page describing record declaration-related errors/warnings and how to resolve them.