Reworked the core examples to exclude contributed examples.

examples/Ex020_Rounding_Corners_with_Fillets.py

@@ -0,0 +1,13 @@
+import cadquery as cq
+
+# Create a plate with 4 rounded corners in the Z-axis.
+# 1.  Establishes a workplane that an object can be built on.
+# 1a. Uses the X and Y origins to define the workplane, meaning that the
+#     positive Z direction is "up", and the negative Z direction is "down".
+# 2.  Creates a plain box to base future geometry on with the box() function.
+# 3.  Selects all edges that are parallel to the Z axis.
+# 4.  Creates fillets on each of the selected edges with the specified radius.
+result = cq.Workplane("XY").box(3, 3, 0.5).edges("|Z").fillet(0.125)
+
+# Displays the result of this script
+show_object(result)