Consider bootstrap 4 implications

[zkamvar]

There is a lot of discussion buzzing about the R community to switching over to bootstrap 4.

Examples:

r-lib/pkgdown#1409
rstudio/rmarkdown#1260
rstudio/rmarkdown#1260 (comment)

It may be worthwhile just to go with bootstrap4 from the get-go and see if we can tool around that.

[zkamvar]

I knew this would fail, but the first test with swapping out the script sources for bootstrap 4 and jQuery gives this monstrosity:

[fmichonneau]

bootstrap 5 is coming out soon, when using it with our own version of the template, we need to look into whether we should skip version 4 altogether.

[zkamvar]

I think we may want to start with 4 and migrate to 5 once the RMarkdown verse is also there. At the moment, they are still on 4, but are considering 5: rstudio/bslib#82

One of the things I've been eyeing is using {bslib} and a custom RMarkdown template to control the theming. In theory, this should allow people to more easily customize the template for their use.