Nav links at top do not work

[jasongrout]

Despite the book docs saying that urls can be full external urls, I see that the jupyter.org, Jupyter Docs, and Governance nav links seem to be treated as relative urls. The Jupyter.org, Jupyter Docs, and Governance links in the nav bar

lead to https://jupyter.org/governancehttps://jupyter.org, https://jupyter.org/governancehttps://docs.jupyter.org, etc.

CC @choldgraf