svg->png conversion in doc build

[Graham Percival]

We need png versions of the svg images in Documentation/pictures/.
The current web branch stores pngs in git (see site/graphics/).
This is obviously my preference, since it simplifies the build
process.

If that's not acceptable, then could somebody add a rule to
Documentation/pictures/GNUmakefile to generate appropriately-sized
pngs from those svgs?  (whenever I try fumbling around with
imagemagick or inkscape, I get a full-page png with tons of blank
space)

Cheers,
- Graham