Change file extension from .xml to .json in docs as PowerShell defaults to json

[emelbaer]

PR Summary

Updated documentation to reflect the change from .xml to .json file extension for the Export-StartLayout cmdlet.

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

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[learn-build-service-prod]

Learn Build status updates of commit 9ab2518:

✅ Validation status: passed

File	Status	Preview URL	Details
docset/winserver2025-ps/StartLayout/Export-StartLayout.md	✅Succeeded	View (WindowsServer2025-ps)

For more details, please refer to the build report.