Change box order, make diagnostic decision support more general

[jaclyn-taroni]

Stacked on #242 - I am about to head into a bunch of meetings, so I figured I'd file this draft for discussion.

• To me, it makes more sense to have the box that introduces common tasks as the first box that we reference in the introduction, where we give a few examples of supervised and unsupervised learn. The box with common tasks becomes an extension of those ideas. I've reordered the files and mentions of boxes in the text as well.
  • Box 3 ML tasks -> Box 1
  • Box 1 experimental design -> Box 2
  • Box 2 definitions -> Box 3
• I changed "Diagnostic Decision Support" to "Identifying patients with rare diseases" in Box 1a (Box 3a -> Box 1a). This strikes me as a more general way of talking about the same tasks, and I was not familiar with this terminology.

[jaclyn-taroni]

Okay I will merge this into the other branch!