Add "Update Windows App SDK" in "Get started WinUI 3 (Win App SDK)"

[mikehoffms]

Rendered article section for review:

• Get started with WebView2 in WinUI 3 (Windows App SDK) apps > Step 4: Update or install the Windows App SDK
  • https://review.learn.microsoft.com/microsoft-edge/webview2/get-started/winui?branch=pr-en-us-3297#step-4-update-or-install-the-windows-app-sdk
  • Before/Live
  • ⁠Clarified installation steps.
    • Made all steps self-contained, specific, smooth, robust, easy-to-follow.
    • Brought step 1.1 inline instead of jumping to other docset.
  • Updated all screenshots.
    • Added screenshots of install steps.
    • Changed from light mode to dark mode (defaults).
    • Changed from bing photo to blank bing.
    • Showed new name of project.
  • Improved entire article.
    • Clarified what we're doing in each add-code step.
    • Added intro summary of major steps.
    • Moved informational section "WinAppSDK supports custom WebView2 environments" from an action-steps section down to below all the action-steps sections.

Related PR in WebView2Samples repo: PR 257 (Replace WinUI3GetStarted 2020 project by result of latest project template)

AB#54470523

[sivMSFT]

LGTM - PM

[mikehoffms]

LGTM - Content Developer (Writer/Editor)

[learn-build-service-prod]

Learn Build status updates of commit ee122cd:

✅ Validation status: passed

File	Status	Preview URL	Details
microsoft-edge/webview2/concepts/navigation-events.md	✅Succeeded	View
microsoft-edge/webview2/get-started/get-started.md	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/blank-winui3-app.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/config-new-proj.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/create-project-winui3-desktop.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/enable-developer-mode.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/finished-app.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/getting-started-bing.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/getting-started-part-3.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/getting-started-script.png	✅Succeeded	n/a (file deleted or renamed)
microsoft-edge/webview2/get-started/winui-images/installer-winappsdk.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/new-winui3-project.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/nuget-install-winappsdk.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui-images/nuget-installed-tab.png	✅Succeeded	View
microsoft-edge/webview2/get-started/winui.md	✅Succeeded	View

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