Add flowcharts to aid explanations of processes

[avivace]

E.g. Object Priority and Conflicts, currently being reworked by #331

[ISSOtm]

I think that it's a better idea to generate those flowcharts from more usable data, rather than drawing them by hand (either using an editor program, or by hand like a madman).

A quick search yields diagrams, powered by flowchart.js, which looks serviceable enough. [NB: I also found mermaid and mdbook-mermaid, but I rejected them because they run client-side.]

Now, I'm wary of introducing too many build dependencies into Pan Docs, since we already have Rust (mdBook + "plug-ins") and Python (MBC graph generation), and this would add Node/JS to the list.

• Is this acceptable?
• If not:
  • Do we hand-roll our own generator in one of these two languages? (Pro: a custom generator may play better with #330.)
  • Do we replace one of our current languages with JS? (Given that getting rid of Rust would require switching to another infrastructure again, the Python script would likely have to be ported.)