add favicon

Created by: sqs

fix https://github.com/sourcegraph/about/issues/324

I considered using a slightly different icon to distinguish the handbook from Sourcegraph.com and the nicely designed parts of about.sourcegraph.com, but I decided against it because I think the line will get blurry between the handbook and the rest of about.sourcegraph.com (eg what is https://about.sourcegraph.com/direction? handbook or about.sourcegraph.com?).