Add areaplot example to gallery

Not sure this is the right way to add example to the gallery but hopefully this is helpful :)

src/examples.jl

@@ -1273,6 +1273,17 @@ const _examples = PlotExample[
             end
         )],
     ),
+    PlotExample( # 58
+        "Stacked area chart",
+        """
+        `areaplot` draws stacked area plots.
+        """,
+        [:(
+            begin
+                areaplot(1:3, [1 2 3; 7 8 9; 4 5 6], seriescolor = [:red :green :blue], fillalpha = [0.2 0.3 0.4])
+            end
+        )],
+    ),
 ]
 
 # Some constants for PlotDocs and PlotReferenceImages