Transparent logo

The bund logo in api doc is ugly since its background color is white and doc background color is ligth grey. I created a transparent png intead of the jpg.

This is compliant with the ['Identité visuelle de l'Administration fédérale suisse' Handbuch](https://www.google.ch/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB8QFjAA&url=http%3A%2F%2Fwww.bk.admin.ch%2Fthemen%2F02268%2F02385%2Findex.html%3Flang%3Dfr%26download%3DNHzLpZeg7t%2Clnp6I0NTU042l2Z6ln1ae2IZn4Z2qZpnO2Yuq2Z6gpJCFeYJ5g2ym162epYbg2c_JjKbNoKSn6A--&ei=XxN_VLDuOMXVygPv04CABQ&usg=AFQjCNGKvdC3x0U4kpa6PREzh_c00TrRKA&sig2=8PBsJBpmofSIrko7S-gi1g&bvm=bv.80642063,d.bGQ&cad=rja)
(see page 20).