Autogenerate entity relation diagram for docs

[epompeii]

Parse the schema.rs generated by diesel and then output a format understood by a diagramming tool:

• https://github.com/mermaid-js/mermaid
• https://github.com/BurntSushi/erd
• https://github.com/davechallis/erd-rs

[epompeii]

Mermaid syntax is now supported by GitHub readmes: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams