[Colors] Adding highcontrast brushes (#1401)

This PR adds a `High Contrast` page to the colors overview, and shows
the 8 high contrast brushes that come with WinUI.

This closes: #1217

New high contrast page:

![image](https://github.com/microsoft/WinUI-Gallery/assets/9866362/48aaac45-f3ff-483f-91f9-f95b8ba324f1)



Screenshot from WinUI Visual Library Figma (source):

![image](https://github.com/microsoft/WinUI-Gallery/assets/9866362/1323d9df-fd4a-44de-a5b2-95500911fab0)

WinUIGallery/ControlPages/DesignGuidance/ColorsPage.xaml

@@ -42,6 +42,7 @@
             <x:String>Stroke</x:String>
             <x:String>Background</x:String>
             <x:String>Signal</x:String>
+            <x:String>High Contrast</x:String>
         </ComboBox>
 
         <controls1:SampleThemeListener Grid.Row="2">

WinUIGallery/ControlPages/DesignGuidance/ColorsPage.xaml.cs

@@ -31,6 +31,9 @@ private void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
                 case 4:
                     NavigationFrame.Navigate(typeof(SignalSection));
                     break;
+                case 5:
+                    NavigationFrame.Navigate(typeof(HighContrastSection));
+                    break;
             }
         }