Updates to work with {pkgdown} >= 2.1.0

[joshwlambert]

This PR makes several updates to ensure maximum compatibility with the newest {pkgdown} release (2.1.0).

• All vignettes now use output: rmarkdown::html_vignette (some vignette were previously using bookdown::html_vignette2), as this reduces complications with {pkgdown} rendering (https://pkgdown.r-lib.org/reference/build_articles.html#output-formats).
• Remove pkgdown: as_is: true from vignettes to ensure math mode renders correctly
• Change development: mode: auto to mode: unreleased to ensure online documentation is always up-to-date with GitHub package (this will be reverted when {simulist} is hosted on CRAN)
• code_folding: show has been removed from all vignettes as it is not supported by {pkgdown} without as_is: true. class.source = 'fold-hide' is also removed from code chunks as a result. The alternative to use <details> (as recommended in {pkgdown} support code folding r-lib/pkgdown#1433) is not equivalent as it also folds the output of the code chunks including plots, therefore it was decided not use it. By default all code is shown including plotting code, which was folded by default in previous versions.
• The bullet point discussing the use of rmarkdown::html_vignette and bookdown::html_vignette2 and its influence on interactive plots (html widgets) has been removed as it is no longer necessary as all vignettes use rmarkdown::html_vignette.
• Manual figure numbering has been removed from vis-linelist.Rmd vignette. All other vignettes no longer have figure numbering due to removing bookdown::html_vignette2 and as_is: true.

[github-actions]

This pull request:

• Adds 0 new dependencies (direct and indirect)
• Adds 0 new system dependencies
• Removes 1 existing dependencies (direct and indirect)
• Removes 1 existing system dependencies

(Note that results may be inaccurate if you branched from an outdated version of the target branch.)