Add JupyterCon banner and jupyter colors

[choldgraf]

This PR does a couple of things:

• Adds an announcement banner for JupyterCon
• Links a CSS file from the Jupyter documentation that we can use for jupyter colors in our docs

[github-actions]

👈 Launch a binder notebook on branch choldgraf/ipywidgets/patch-1

[choldgraf]

the build problems seem to be unrelated to this, I think they are redirect errors?

[jasongrout]

I did a test PR to check if the build problem is systemic, and the build there passed. I'll spend a few minutes looking into this.

[choldgraf]

oops that was a silly mistake from me, I didn't see the pre-existing setup(app), thanks for finding that

[jasongrout]

I think I found the problem. @choldgraf - you added a new setup function in the conf.py, which overwrote the existing setup function. The existing setup function is what triggered building the pages that the redirection error was complaining about.

I just combined the two setup functions.