Document EmitCompilerGeneratedFiles and CompilerGeneratedFilesOutputPath MSBuild properties

[Copilot]

The MSBuild properties reference page was missing documentation for the source generator file output properties mentioned in the linked issue. (GenerateDocumentationFile was already documented.)

Changes

• EmitCompilerGeneratedFiles — new section in "Generated file properties": controls whether Roslyn source generator output is written to disk during build (default: in-memory only)
• CompilerGeneratedFilesOutputPath — new section: specifies the output directory for emitted files; defaults to obj/<config>/<tfm>/generated; includes a note on excluding the path from <Compile> to prevent double-compilation
• Updated the section's property list to include both new entries (alphabetical order)
• Updated ms.date and added ai-usage: ai-assisted to frontmatter

Usage example

<PropertyGroup>
  <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
  <CompilerGeneratedFilesOutputPath>Generated</CompilerGeneratedFilesOutputPath>
</PropertyGroup>

<!-- Prevent double-compilation if path is inside the source tree -->
<ItemGroup>
  <Compile Remove="$(CompilerGeneratedFilesOutputPath)/**/*.cs" />
</ItemGroup>

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Internal previews

📄 File	🔗 Preview link
docs/core/project-sdk/msbuild-props.md	MSBuild reference for .NET SDK projects

[BillWagner]

This LGTM and is ready for final review.

[Copilot]

Pull request overview

This PR updates the .NET SDK MSBuild properties reference to document two Roslyn source generator file output properties that weren’t previously covered.

Changes:

• Adds documentation sections for EmitCompilerGeneratedFiles and CompilerGeneratedFilesOutputPath.
• Updates the “Generated file properties” list to include the new entries.
• Updates frontmatter (ms.date) and adds ai-usage: ai-assisted.