Add Customer to global nav

Adds Customer as a top-level global navigation entry to make it far easier to find documentation for that context.

See: http://add-customer-nav-entry.178.62.207.141.xip.io/ce/README.html

I've tried to keep a rough "how frequently a global nav entry would be needed" order to it. I've assumed that Customer would be a less frequent use case for docs than Administrator, but more frequent than API.

CC @cynthia @amandarueda @tipyn

Somewhat related to: https://gitlab.com/gitlab-org/gitlab-ee/issues/10910