Update File Explorer context menu guidance

[xandfis]

Summary

• document Windows 11 context menu integration through IExplorerCommand
• describe COM and package-manifest registration for packaged and sparse-manifested apps
• remove obsolete Windows 10 file-association walkthrough and retired screenshots

Validation

• Editor diagnostics clean
• git diff --check

[prmerger-automator]

@xandfis : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change. @stevewhims

[prmerger-automator]

@xandfis : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change. @stevewhims

[learn-build-service-prod]

Learn Build status updates of commit d52663c:

✅ Validation status: passed

File	Status	Preview URL	Details
hub/apps/desktop/modernize/images/file-explorer/folder-context-menu.png	✅Succeeded	n/a (file deleted or renamed)
hub/apps/desktop/modernize/images/file-explorer/popup.png	✅Succeeded	n/a (file deleted or renamed)
hub/apps/desktop/modernize/images/file-explorer/resize-file.png	✅Succeeded	n/a (file deleted or renamed)
hub/apps/desktop/modernize/integrate-packaged-app-with-file-explorer.md	✅Succeeded

For more details, please refer to the build report.

[prmerger-automator]

PRMerger Results

Issue	Description
File Change Percent	This PR contains file(s) with more than 30% file change.

[GrantMeStrength]

Good rewrite — technically accurate and a clear improvement over the Windows 10/MSI-era content.

Verified:

• All IExplorerCommand methods match the Win32 API contract
• com:SurrogateServer is the correct COM registration for packaged shell extensions (per the Windows 11 context menu extension guidance)
• desktop4/desktop5 namespace versions and all manifest attribute names are correct
• Sparse-package section is accurate (uap10:AllowExternalContent, mediumIL, win32App)
• ItemType values (*, Directory, Directory\Background) are correct

One nit: front matter is missing author: and ms.author: — I'll add those when porting to the private repo.