Add support for UML class diagrams

[oruebel]

It would be nice to be able to plot format specification as UML class diagrams. Some options for this are:

• Mermaid:
  • https://mermaid-js.github.io/mermaid/#/
  • https://sphinxcontrib-mermaid-demo.readthedocs.io/
• Graphviz:
  • http://www.ffnn.nl/pages/articles/media/uml-diagrams-using-graphviz-dot.php
  • https://www.sphinx-doc.org/en/master/usage/extensions/graphviz.html
• Text UML tools:
  • https://modeling-languages.com/text-uml-tools-complete-list/
• D3 and other JS libs (web-only)
  • https://github.com/hnakamur/d3.js-class-diagram-example
  • https://github.com/dibaunaumh/model
  • https://modeling-languages.com/javascript-drawing-libraries-diagrams/