Publish the C# 13 alpha standard

[BillWagner]

Publish the alpha-v13 standard, retire C# 13 feature speclets, and retarget active proposal links to their normative clauses.

Fixes #55472

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Internal previews

File	Preview link
docs/csharp/language-reference/attributes/general.md	Preview published page
docs/csharp/language-reference/operators/collection-expressions.md	Preview published page
docs/csharp/programming-guide/classes-and-structs/partial-classes-and-methods.md	Preview published page
docs/csharp/specification/overview.md	Preview published page
docs/csharp/specification/toc.yml	Preview published page
docs/csharp/whats-new/csharp-13.md	Preview published page

[Copilot]

Pull request overview

This PR updates the documentation set to publish the C# 13 alpha (early draft) standard content and to retire C# 13 feature speclet references by retargeting them to normative specification clauses.

Changes:

• Retarget C# 13 “What’s new” and related conceptual docs from C# 13 proposal speclets to the corresponding C# standard draft clauses.
• Update the C# specification landing/TOC to reflect the C# 13 early draft.
• Retarget the external standard source branch to alpha-v13 and stop including C# 13 proposal speclets in DocFX configuration.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
docs/csharp/whats-new/csharp-13.md	Updates feature references to point at C# standard draft clauses instead of C# 13 proposal speclets.
docs/csharp/specification/toc.yml	Removes C# 13 proposal speclets from the spec TOC and updates the early draft label to C# 13.
docs/csharp/specification/overview.md	Updates the overview text and date to reflect the C# 13 early draft.
docs/csharp/programming-guide/classes-and-structs/partial-classes-and-methods.md	Retargets partial-member references to the standard clauses for partial properties/indexers.
docs/csharp/language-reference/operators/collection-expressions.md	Retargets “better conversion” reference to the standard clause.
docs/csharp/language-reference/attributes/general.md	Retargets overload resolution priority reference to the standard clause.
docfx.json	Removes C# 13 proposal speclet inclusions/metadata entries and updates standard ms.date metadata.
.openpublishing.publish.config.json	Updates the external standard source branch from alpha-v12 to alpha-v13.

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