Currently if you end up at opendev.org it isn't clear how to start
contributing code. Add a "Get Started" link to the nav bar header to
will send you to the opendev developer docs.
Note we may want to consider updating the developer docs to be a better
landing page or even create a new doc for that, but this is a start.
Change-Id: Iee43e2552c1be2e4cd46f43f050e476f140530ad
e6639c52c1