(Version 0.21.X) Support for additional diagram types

Added software to the Docker image to support Dpic, Gnuplot, MscGen, Nomnoml, Pintora, State Machine Cat (smcat), and TikZ.

Added links to the documentation of diagram types supported by the AsciiDoctor Diagram extension to the AsciiDoc Showroom.

Adjusted the order of examples for supported diagram types in the AsciiDoc Showroom.

Added example code for Dpic, Gnuplot, MscGen, Nomnoml, Pintora, State Machine Cat (smcat), TikZ to the AsciiDoc Showroom.

Version 0.21.1: Fixed the order of examples for supported diagram types in the AsciiDoc Showroom.

Version 0.21.2: Added a deployment diagram of this repository to the README.