use svg instead of png in documentation

Our tarball contains 8MB of PNG files (3.5MB of UML diagrams in the Doxygen doc, and 5.5MB of automata in the org-mode documentation). Michael Roynard suggest we switch to SVG. That would save space, improve the quality and also speedup make dist (no need for optipng anymore).

Doing this for Doxygen is probably easier than for org-mode (where we will have to deal with the Lato font and size issues).