Document .NET 11 badge support for Shell tabs and ToolbarItems

[Copilot]

.NET MAUI 11 Preview 4 adds badge support for Shell tabs and ToolbarItem controls. This updates the docs to surface the new capability in both feature documentation and What's New content.

• What's New

  • Added a .NET 11 Preview 4 entry for Shell tab and toolbar item badges.
  • Linked to the detailed Shell tab and ToolbarItem badge docs.
  • Referenced the upstream MAUI implementation PRs.

• Feature docs

  • Added hidden screenshot placeholders near the Shell tab and ToolbarItem badge examples for future image assets.

Example badge usage:

<ToolbarItem Text="Inbox"
             IconImageSource="inbox.png"
             BadgeText="3"
             BadgeColor="Red"
             BadgeTextColor="White" />

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/shell/tabs.md	docs/fundamentals/shell/tabs
docs/user-interface/toolbaritem.md	docs/user-interface/toolbaritem
docs/whats-new/dotnet-11.md	docs/whats-new/dotnet-11

[learn-build-service-prod]

Learn Build status updates of commit 6360483:

✅ Validation status: passed

For more details, please refer to the build report.

[learn-build-service-prod]

PoliCheck Scan Report

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[learn-build-service-prod]

Learn Build status updates of commit 9465cc1:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/fundamentals/shell/tabs.md	✅Succeeded	View (>=net-maui-8.0)
docs/user-interface/toolbaritem.md	✅Succeeded	View (>=net-maui-8.0)
docs/whats-new/dotnet-11.md	✅Succeeded	View (>=net-maui-8.0)

For more details, please refer to the build report.