Add JupyterCon banner and add 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 on branch jupyter/notebook/choldgraf-patch-1

[jtpio]

Thanks!

Looks like it needs a quick lint pass, otherwise sounds like a good idea 👍

[choldgraf]

OK I think I figured it out and it should work now, let's see if tests pass

[Zsailer]

Thanks!