Add reference to the widgets sample in the documentation

Add reference to the description of the widgets sample wherever this
sample is mentioned in the documentation.

interface/wx/slider.h

@@ -140,7 +140,7 @@
     (In all these cases the EVT_SCROLL_THUMBRELEASE event does not happen).
     In short, the EVT_SCROLL_CHANGED event is triggered when scrolling/ moving
     has finished independently of the way it had started.
-    Please see the widgets sample ("Slider" page) to see the difference between
+    Please see the @ref page_samples_widgets ("Slider" page) to see the difference between
     EVT_SCROLL_THUMBRELEASE and EVT_SCROLL_CHANGED in action.
 
     @library{wxcore}