[docu/MeL] Add further docu for ExtractSurface.

web/content/docs/tools/meshing/extract-surface/index.pandoc

@@ -33,3 +33,10 @@ Extracted top, bottom and side surfaces:
 - top `ExtractSurface -i Input.vtu -o TopSurface.vtu`
 - bottom `ExtractSurface -i Input.vtu -o BottomSurface.vtu -x 0 -y 0 -z 1`
 - side `ExtractSurface -i Input.vtu -o SideSurface.vtu -x 1 -y 1 -z 0 -a 45`
+
+![](CubeFrontRightBackLeft.png)
+
+The figure shows the extracted front, right, back, and left surfaces from the
+cube that are colored by the corresponding subsurface material id. The material
+ids transformed to the surfaces can be used for further boundary condition
+preparations for instance employing paraviews threshold filter.