doc: Switch from png to svg for graph images

This substantially reduces the image sizes.

doc/Doxyfile.in

@@ -1798,7 +1798,7 @@ DIRECTORY_GRAPH        = YES
 # HTML_FILE_EXTENSION to xhtml in order to make the SVG files
 # visible in IE 9+ (other browsers do not have this requirement).
 
-DOT_IMAGE_FORMAT       = png
+DOT_IMAGE_FORMAT       = svg
 
 # If DOT_IMAGE_FORMAT is set to svg, then this option can be set to YES to
 # enable generation of interactive SVG images that allow zooming and panning.