Add visualization debug option to the docca build

[evanlenz]

I am hoping to eventually get a visualization debug option working on the docca build using xslt-visualizer (which itself has some pending optimizations that will affect the output, so it's not quite ready yet). Here's a basic demo. Here is some documentation that provides an overview in relation to docca. And here's a sample output for Beast from a while back. The "doc_visualized" build was using bash scripts and got removed from the docca project. I would eventually like to make it bjam-friendly and get it back into docca as a build option. Once I get the latest xslt-visualizer changes checked in, we should be good to explore that.