Allow users to switch between dark and light mode (#2630)

cf. https://pkgdown.r-lib.org/articles/customise.html?q=dark%20#light-switch

_pkgdown.yaml

@@ -2,6 +2,7 @@ url: https://lintr.r-lib.org
 
 template:
   bootstrap: 5
+  light-switch: true
   includes:
     in_header: |
       <script defer data-domain="lintr.r-lib.org,all.tidyverse.org" src="https://plausible.io/js/plausible.js"></script>