Adopt wide jupyterhub logo in design assets

[choldgraf]

We use this logo in a few places, in-part to follow similar patterns in the JupyterLab ecosystem. It feels like it fits the "wide" logo niche nicely. What do folks think about adding this to our docs so others know it's an acceptable use of the logo/brand?

[minrk]

sounds good to me!

[rgaiacs]

Looks good to me.