Rendering mermaid diagrams to pdf causes random time-outs

[sebffischer]

Other people have experienced similar random timeouts as we have (https://github.com/quarto-dev/quarto-actions/issues/45#issuecomment-1798392018). Until the problem is identified and there is a proper fix for this, we should not include mermaid diagrams in the book. As a workaround, we can still create mermaid diagrams, convert them to svg / png and then include them using include_multi_graphics(). Once this issue is solved, we can use the mermaid chunks again.

I have done this for the existing mermaid diagrams here: https://github.com/mlr-org/mlr3book/pull/794