Stop manually building images out of dot files

The first dot files we added way back in the 2.0 release are managed
manually: we build the .pngs ourselves, track them in .gitingore,
etc. Since then, we have switched to allow sphinx to build/manage them
itself, so it makes sense to cleanup the build system and covert
everything to this simpler method.

This patch simply converts from "image: x.png" to "graphviz: x.dot",
as an externally-referenced dot file, and removes all the custom
makefile rules for png conversion. The next patch will bring the
conversion further (by inlining then).

Additionally, while checking that the patch doesn't change the html
output, I saw that the use of "shape=diamond" with non-trivial labels
results in very ugly diagrams; so we split the question in such
decision boxes to make them more reasonably looking.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>