Provide better navigation between the docs and the console

[cnunciato]

When you navigate from the console to the docs (e.g., by clicking Docs in the top nav), it's unclear how to get back to the console. The Sign In link will do it, but it's poorly named because in this scenario, you're already signed in -- and we actually know this, because we have access to a cookie with your username in it. We should fix this. (Also reported in an onboarding doc.)