docs(header): replace logo.png with logo.svg

The current logo looks awful on high-density displays.  SVG is a
better choice because it can scale to any resolution without
increasing file size.

Amending #2775 to add support for IE 8 by falling back to existing PNG
with img.onerror

Using relative URLs as directed by @btford and @petebacondarwin.

(commit by Brenton Simpson - @appsforartists)

Closes #2874