* Replace previous TX Overview image with a simplified illustration.
The previous overview image was commented out in pull #566 / commit
6afc683. New image incorporates suggestions by @saivann (thanks!)
* 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
