Add footer with required links

[ryanlovett]

I think we should add a footer with links to

• Accessibility
• Nondiscrimination
• Privacy

There's an example footer at https://jupyter-book.github.io/example-landing-pages/.

It may also help with #25 in some small way.

[paciorek]

fine by me

[ryanlovett]

I added something that approximates the department's site, but now I'm thinking we should choose something from the newer branding colors along with some imagery.