Fix scrambled output in the introduction vignette

[strohne]

In https://strohne.github.io/volker/articles/introduction.html the codebook() function output is ugly:

[strohne]

Done

[strohne]

Well, not done yet....why do the vignettes render differently in the local system compared to the GitHub actions workflow?

[strohne]

Fixed. In the options, we need to set comment to "#>" and collapse to TRUE.

knitr::opts_chunk$set(
  comment = "#>",
  echo = TRUE,
  message = FALSE,
  collapse=TRUE
)

See ddsjoberg/starter#25 and r-lib/pkgdown#1843