Add new "Steps Types" example/test plot 53

This new example/test plot showcases the various step-like
`seriestype`s: `:steppre`, `:stepmid`, and `:steppost`.

src/examples.jl

@@ -1119,6 +1119,25 @@ const _examples = PlotExample[
             quiver(x,y,z, quiver=(u,v,w))
         end]
     ),
+    PlotExample( # 53
+        "Step Types",
+        "A comparison of the various step-like `seriestype`s",
+        [
+            :(
+              begin
+                  x = 1:5
+                  y = [1, 2, 3, 2, 1]
+                  default(shape=:circle)
+                  plot(
+                       plot(x, y, markershape=:circle, seriestype=:steppre, label="steppre"),
+                       plot(x, y, markershape=:circle, seriestype=:stepmid, label="stepmid"),
+                       plot(x, y, markershape=:circle, seriestype=:steppost, label="steppost"),
+                       layout=(3,1)
+                      )
+              end
+            ),
+        ],
+    ),
 ]
 
 # Some constants for PlotDocs and PlotReferenceImages