Consider using mermaid diagrams instead of graphviz

[Bisaloo]

E.g., to replace:

flowchart LR
    S -->|"infection\n(transmission rate β)"| E
    E -->|"onset of infectiousness\n(infectiousness rate α)"| I
    I -->|"recovery\n(recovery rate γ)"| R

flowchart LR
    S -->|"infection\n(transmission rate β)"| E
    E -->|"onset of infectiousness\n(infectiousness rate α)"| I
    I -->|"recovery\n(recovery rate γ)"| R

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This will allow a better integration with varnish (carpentries/varnish#125). In particular, colour theme will be respected & text can be selected.