Add new footer to rest of the site

What does this merge do?

Here we add the new footer design to the rest of the docs site.

A tiny MVC for #655 (closed) based on the Simple design.

Screenshots

Large	Medium	Small	Xtra Small

Notes && Follow ups

1. Redesign 404 page to be mobile responsive => Candidate for the new @gitlab/ui integration?
2. If we get the green light, remove the old footer.html 😄
3. Consider making the footer links a dynamic list to support multiple versions