Unify font of docs and home page

[jamesplease]

Right now the home page uses the very-bubbly Questrial font for everything, but our docs are using the elegant Helvetica Neue for headers and Times for the text. We should be more consistent.

If my adjectives didn't give me away, I'm not a fan of Questrial. It seems a bit too playful for me. I know @jdaudier is a fan, though. I'll make a separate issue for this.

[samccone]

cc @lupinskij