Add a figure for Garland-Heckbert simplification

Surface_mesh_simplification/doc/Surface_mesh_simplification/Doxyfile.in

@@ -1,3 +1,5 @@
 @INCLUDE = ${CGAL_DOC_PACKAGE_DEFAULTS}
 
 PROJECT_NAME = "CGAL ${CGAL_DOC_VERSION} - Triangulated Surface Mesh Simplification"
+
+HTML_EXTRA_FILES           =  ${CGAL_PACKAGE_DOC_DIR}/fig/sappho.jpg

Surface_mesh_simplification/doc/Surface_mesh_simplification/Surface_mesh_simplification.txt

@@ -188,6 +188,17 @@ face normals). This variance naturally deteriorates the tightness of the result,
 hand it enables creating more uniform triangulations and the approach is more tolerant to noise,
 while still maintaining feature sensitivity.
 
+\cgalFigureAnchor{SurfaceMeshSimplification_GH}
+<center>
+<img src="sappho.jpg" style="max-width:70%;"/>
+</center>
+\cgalFigureCaptionBegin{SurfaceMeshSimplification_GH}
+<a href="https://ten-thousand-models.appspot.com/detail.html?file_id=72286">Sappho's Head</a> model (leftmost, 34882 vertices).
+From left to right, simplified output (1745 vertices) and symmetric Hausdorff distance
+for the four Garland-Heckbert variations: plane (0.217912), probabilistic
+plane (0.256801), triangle (0.268872), and probabilistic triangle (0.490846).
+\cgalFigureCaptionEnd
+
 \subsection Surface_mesh_simplificationCostStrategyPolicies Cost Strategy Policies
 
 The cost strategy used by the algorithm is selected by means of three policies: