Add docs on ExtendedLayout

[BillWagner]

Fixes #51746

Add description of the ExtendedLayoutAttribute in the language attributes section.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/attributes/general.md	Miscellaneous attributes interpreted by the C# compiler
docs/csharp/language-reference/attributes/pseudo-attributes.md	"Attributes interpreted by the compiler: Pseudo-attributes"

[Copilot]

Pull request overview

This PR adds documentation for the ExtendedLayoutAttribute, a new compiler-recognized attribute in .NET 11 that allows developers to specify the ExtendedLayout flag in type metadata. The attribute is applied in source code as an alternative to [StructLayout(LayoutKind.Extended)], which cannot be written directly in C# code.

Changes:

• Added ExtendedLayoutAttribute section to the general attributes documentation with description and compatibility warnings
• Added warning in pseudo-attributes documentation about StructLayoutAttribute incompatibility with ExtendedLayout
• Updated ms.date metadata to reflect the documentation changes

Reviewed changes

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

File	Description
docs/csharp/language-reference/attributes/general.md	Added ExtendedLayout attribute section with API references, description, and IMPORTANT callouts for incompatible attributes (StructLayoutAttribute and InlineArray). Also added the attribute to the table of contents list at the top.
docs/csharp/language-reference/attributes/pseudo-attributes.md	Added IMPORTANT callout warning that StructLayoutAttribute cannot be combined with ExtendedLayout, creating cross-reference between the two attribute documentation pages.