add a PreFigure sidebyside into the sample-article

[davidaustinm]

This adds two annotated PreFigure diagrams in a sidebyside in the sample article. I don't think it's ready to be merged, but I wanted to illustrate some ways in which PreTeXt and diagcess are not getting along.

1. The @widths and @margins attributes of the sidebyside seem not to be fully respected, and the result of changing the values of those attributes doesn't seem entirely predictable.
2. The relative size of the two diagrams changes when the dimensions of one of the SVGs changes, which I don't believe should happen. The version here looks alright (after a bit of fiddling), but changing @dimensions="(300,200)" from (200,200) in the first diagram will change the relative size of the two diagrams and sometimes one diagram will bleed over into the other.
3. I'm wondering if the navigation instructions could be placed in the margin. It's a relatively long piece of text "Diagram Exploration Keyboard Controls" and when it breaks into two lines under one of the diagrams, the vertical alignment is affected. In a sidebyside, we shouldn't need it twice.

[davidaustinm]

PS: Grab the prefig nightly before compiling.