chore: upgrade pkgdown to bootstrap 5

[fh-mthomson]

Intent

Address pkgdown aspect of #275

Approach

When setting up pkgdown in a new package, via usethis::use_pkgdown() programmatically adds:

template:
  bootstrap: 5

Which should be minimally sufficient to enable search by default, from what I've seen in developing other packages.

Related docs:

• pkgdown 2.0.0 and bootstrap 5
• bootstrap 5
• search - automatically added via `build_site()

Testing

I'm unable to run pkgdown::build_site() locally, given vignettes that require certain test artifacts to be accessible from a Connect instance. I'll take a look at the resulting CI logs, but a bit limited in being able to test more than that, from what I can tell.

Checklist

• Does this change update NEWS.md (referencing the connected issue if necessary)?
• Does this change need documentation? Have you run `document()?

[toph-allen]

This change looks good. CI passed in #310. I'll take it from here and get it merged.