Add snippet for Xamarin Screenshot (#5273)

Author: lucas-zimerman

src/includes/enriching-events/attach-screenshots/dotnet.xamarin.mdx

@@ -0,0 +1,7 @@
+```csharp
+SentryXamarin.Init(options =>
+{
+    options.Dsn = "___PUBLIC_DSN___";
+    options.AttachScreenshots = true;
+});
+```