Revise TX Overview Image

Preview: http://dg0.dtrt.org/en/developer-guide#transactions

* Replace previous TX Overview image with a simplified illustration.
  The previous overview image was commented out in pull #566 / commit
  6afc683

* Add new illustration showing the specifics of outputs and inputs (and
  how they're related). This replaces most of the detail lost by using a
  simplified overview image

* Add two short paragraphs briefly describing the ouput and input
  features, leading into the existing detailed P2PKH description