Provide logos in PNG format

[jimmythompson]

Some people won't be able to make use of an SVG, we should also provide PNGs.

[PeterJCLaw]

Inkscape can render PNGs from SVGs; lots of SR repos already use this via a Makefile, thus enabling easy creation of the PNGs while avoiding them possibly getting out of date.

[PeterJCLaw]

Another alternative here would be to have a CI task which builds the PNGs and makes them available somewhere at a variety of resolutions.