fix(vignettes): reduce vignette sizes

by using output: `rmarkdown::html_vignette`

Refs #71
2 jobs for 73-plot-griwrm-mermaid-code-is-displayed-with-the-diagram in 4 minutes and 43 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Checks
passed check #242631
docker

00:04:43

passed test_all #242630
docker

00:01:04