Replace favicon.ico with an actual ICO file

Previous it actually was a PNG.

Before:
% file docs/images/favicon.ico
docs/images/favicon.ico: PNG image data, 32 x 32, 8-bit/color RGBA, non-interlaced

After:
% file docs/images/favicon.ico
docs/images/favicon.ico: MS Windows icon resource - 5 icons, -128x-128, 32 bits/pixel, 64x64, 32 bits/pixel

I converted using:
% convert logo.png -define icon:auto-resize=128,64,48,32,16 favicon.ico

Signed-off-by: Mattia Rizzolo <mattia@mapreri.org>