doc: Add preview image on doc.

Sources/CropImage/DefaultControlsView.swift

@@ -34,7 +34,7 @@ public struct DefaultControlsView: View {
                         .padding(.vertical, 3)
                         .background(
                             RoundedRectangle(cornerRadius: 5, style: .continuous)
-                                .fill(.white)
+                                .fill(.background)
                         )
                 }
                 .buttonStyle(.plain)
@@ -59,7 +59,7 @@ public struct DefaultControlsView: View {
                         .labelStyle(.iconOnly)
                         .padding(1)
                         .background(
-                            Circle().fill(.white)
+                            Circle().fill(.background)
                         )
                 }
                 .buttonStyle(.plain)

Sources/CropImage/Documentation.docc/Documentation.md

@@ -10,6 +10,8 @@ Supports iOS 14.0 and above, or macOS Ventura 13.0 and above.
 - Use `ImageRenderer` to render the cropped image, when possible
 - Very lightweight
 
+![Preview on macOS](macos)
+
 ## Topics
 
 ### Views