docs/website: add favicon.png

The website advertises a favicon in PNG format, but we don't actually
provide one. From docs/website/header.html:

<link rel="shortcut icon" href="images/favicon.png">

So add one. Generated by:

convert -geometry 32x32!  docs/website/images/logo_small.png \
  docs/website/favicon.png

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>