Create favicon for GitLab Handbook

Problem to solve

It is difficult to scan the favicon of GitLab related pages and recognize which tabs relate to the handbook. CleanShot_2020-05-22_at_13.50.44_2x

To make it easier to identify which tabs are GitLab Handbook pages we should design a favicon that represents the Handbook.

Prior art

The only reference I could find to any sort of logo associated with the GitLab handbook is in the form of a social media share image: https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/source/images/tweets/handbook-gitlab.png image

Inspiration

@jeldergl has undertaken a similar task to create a favicon for the GitLab Docs project: https://gitlab.slack.com/archives/C03MSG8B7/p1590080678465500 D