New front-page "hero" image

In #9 we had a bunch of discussion about a new hero image for the frontpage:

https://github.com/mlpack/mlpack.org/pull/9#discussion_r349894124
https://github.com/mlpack/mlpack.org/pull/9#issuecomment-557878121
https://github.com/mlpack/mlpack.org/pull/9#issuecomment-557923717

This issue is a placeholder so that we don't forget about those discussions. :)