docs: add favicon to the website

docs: add favicon to the website

favicon.png is just gears.png resized to 64x64, and favicon.ico is
generated using this command, adapted from the ImageMagick example [1]:

  $ convert favicon.png -background black \
      \( -clone 0 -resize 16x16 \) \
      \( -clone 0 -resize 32x32 \) \
      \( -clone 0 -resize 48x48 \) \
      \( -clone 0 -resize 64x64 \) \
      -delete 0 -alpha off -colors 256 favicon.ico

We could edit every html page to add `<link rel="icon" href="favicon.ico" />`,
but there's not much point as pretty much every browser will pick it up
automatically if the file is named `favicon.ico` and is in the root folder.

[1] http://www.imagemagick.org/Usage/thumbnails/#favicon

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>