Added snapshots for getting started guides

Author: peiche-jessica

GettingStartedGuides/WPF_GettingStarted/README.md

@@ -10,3 +10,5 @@ urlFragment: webview2-wpf-getting-started
 # Getting Started with WebView2 in WPF
 
 This sample relates to the [Getting started with WebView2 in WPF](https://docs.microsoft.com/microsoft-edge/webview2/gettingstarted/wpf) doc.
+
+![sample snapshot](https://raw.githubusercontent.com/MicrosoftDocs/edge-developer/master/microsoft-edge/webview2/gettingstarted/media/wpf-gettingstarted-app.png)

GettingStartedGuides/Win32_GettingStarted/README.md

@@ -10,3 +10,5 @@ urlFragment: webview2-win32-getting-started
 # Getting Started with WebView2 for Win32 apps
 
 This sample relates to the [Getting started with WebView2 for Win32 apps](https://docs.microsoft.com/microsoft-edge/webview2/gettingstarted/win32) doc.
+
+![sample snapshot](https://raw.githubusercontent.com/MicrosoftDocs/edge-developer/master/microsoft-edge/webview2/media/bing-window.png)

GettingStartedGuides/WinUI3_GettingStarted/README.md

@@ -10,3 +10,5 @@ urlFragment: webview2-winui3-getting-started
 # Getting Started with WebView2 in WinUI3
 
 This sample relates to the [Getting started with WebView2 in WinUI3](https://docs.microsoft.com/microsoft-edge/webview2/gettingstarted/winui) doc.
+
+![sample snapshot](https://raw.githubusercontent.com/MicrosoftDocs/edge-developer/master/microsoft-edge/webview2/gettingstarted/media/winui-gettingstarted-bing.png)