Update Snackbar and Toast documentation to clarify Windows packaging …

[ne0rrmatrix]

This pull request updates the .NET MAUI documentation to clarify that both Snackbar and Toast notifications on Windows require a packaged (MSIX) app. It adds important warnings and instructions to help developers avoid runtime errors and understand the platform requirements. The most important changes are grouped below.

Platform requirements and warnings:

• Added explicit [!IMPORTANT] notes to the snackbar.md and shared setup documentation, stating that Snackbar and Toast on Windows require a packaged (MSIX) app because the underlying AppNotificationManager API is only available with packaged app identity. Provided guidance for unpackaged apps and described the exception and diagnostic behavior. [1] [2]
• Updated the setup instructions to clarify that handler registration for Snackbar and Toast is only performed in packaged apps; in unpackaged apps, registration is skipped and a diagnostic message is logged.
• Added a note to the Toast documentation highlighting the packaged app requirement on Windows.

Documentation clarity:

• Improved the language in setup steps to consistently refer to both Snackbar and Toast and to clarify the enabling process for Windows.
• Minor formatting and sectioning improvements for better readability in the Toast documentation.

[learn-build-service-prod]

PoliCheck Scan Report

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[learn-build-service-prod]

Learn Build status updates of commit e28b164:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/maui/alerts/snackbar.md	✅Succeeded	View
docs/maui/alerts/toast.md	✅Succeeded	View
docs/maui/includes/toast-snackbar-setup.md	✅Succeeded	View

For more details, please refer to the build report.