Update default navbar structure (#2595)

vignettes/customise.Rmd

@@ -45,7 +45,7 @@ The following sections show you how.
 
 Please note that pkgdown's default theme has been carefully optimised to be accessible, so if you make changes, make sure that to also read `vignette("accessibility")` to learn about potential accessibility pitfalls.
 
-### Light switch
+### Light switch {#light-switch}
 
 You can provide a "light switch" to allow your uses to switch between dark and light themes by setting the `light-switch` template option to true:
 
@@ -257,10 +257,10 @@ This is the default structure:
 navbar:
   structure:
     left:  [intro, reference, articles, tutorials, news]
-    right: [search, github]
+    right: [search, github, lightswitch]
 ```
 
-It makes use of the the six built-in components:
+It makes use of the the following built-in components:
 
 -   `intro`: "Get Started", which links to a vignette or article with the same name as the package[^dots].
 -   `reference`, if there are any `.Rd` files.
@@ -269,6 +269,7 @@ It makes use of the the six built-in components:
 -   `news`, if `NEWS.md` exists.
 -   `search`, the search box (see `?build_search` for more details).
 -   `github`, a link to the source repository (with an icon), if it can be automatically determined from the `DESCRIPTION`.
+-   `lightswitch`, a ["light switch"](#light-switch) to select light mode, dark mode, or auto mode.
 
 [^dots]: Note that dots (`.`) in the package name need to be replaced by hyphens (`-`) in the vignette filename to be recognized as the intro. That means for a
     package `foo.bar` the intro needs to be named `foo-bar.Rmd`.