add missing logo.svg

Not sure if the logo was intended to be removed or just out by accident. If it's the former, we should probably remove [the reference][1] in the stylesheet.

[1]:https://github.com/darrinhenein/mozilla-docs/blob/d3f5a43981c7147823725b8a917f04b87a3417a8/contents/css/style.scss#L21