fix: R CMD BUILD of vignettes

[akselthomsen]

This PR fixes some R CMD Build issues related to the vignettes:

• whirl.Rmd is now rendered using the knitr::rmarkdown_notangle Vignette engine to make sure the vignette is not rerun as an R program using purl(). See more in this comment: Setting eval = FALSE globally doesn't comment R code when purl()-ing yihui/knitr#2103 (comment).

• example.Rmd has been transformed to an article instead of a vignette, so that it is not build when running R CMD Build/Check commands.

[github-actions]

Github pages

Review the pkgdown webpage for the PR here

[github-actions]

Code coverage

Name	Coverage (%)
whirl	59.88
R/approvedpkgs.R	0.00
R/custom_logging.R	63.04
R/enrich_input.R	100.00
R/internal_run.R	100.00
R/mdformats.R	0.00
R/normalize_with_base.R	100.00
R/pb_script.R	43.75
R/quarto.R	95.24
R/read_regexp.R	93.33
R/render_summary.R	94.12
R/renv.R	0.00
R/run.R	100.00
R/session.R	79.86
R/status.R	91.30
R/strace.R	0.00
R/util_queue_summary.R	100.00
R/whirl_queue.R	94.12
R/whirl_r_session.R	82.11
R/zzz.R	0.00

[falgreen]

Looks good