Support for Asciidoctor diagrams

[lcawl]

@geekpete first drew my attention to https://asciidoctor.org/docs/asciidoctor-diagram/

I've played with it recently in some Asciidoctor-only landing page prototypes, however, and think it's quite useful to have diagrams (particularly for workflows) that can be easily updated within the body of our docs.

The examples I played with used the "ditaa" style (http://ditaa.sourceforge.net/).

Can this be integrated into our existing builds, or do we have to generate them via asciidoctor and then embed the output manually? Hopefully the former is feasible!