Update pkgdown (#159)

* Update package file

Needed to make changes to fix this error from
`pkgdown::build_site()`:

```
! All topics must be included in reference index
✖ Missing topics: ggthemes
ℹ Either add to _pkgdown.yml or use @Keywords internal
ℹ See `$stdout` for standard output.
```

Followed the lead of the ggplot2 package:

- add `@keywords internal`
- Let roxygen2 autogenerate the entire package page.

* Regenerate docs

* Update _pkgdown.yml

---------

Co-authored-by: olivroy <52606734+olivroy@users.noreply.github.com>

R/ggthemes-package.R

@@ -1,17 +1,12 @@
-#' ggthemes
-#'
-#' This package contains extra themes, scales, and geoms, and
-#' functions for and related to \pkg{ggplot2}. See
-#' \url{https://jrnold.github.io/ggthemes/} for documentation.
-#'
-#' @name ggthemes
-#' @docType package
+#' @keywords internal
+"_PACKAGE"
+
 #' @importFrom ggplot2 ggplot theme ggproto aes draw_key_path
 #' @importFrom grid grobTree grobName gTree gList segmentsGrob gpar
 #' @importFrom scales manual_pal div_gradient_pal seq_gradient_pal
 #' @importFrom graphics abline axis text points
 #' @importFrom methods hasArg as
 #' @importFrom graphics par
-"_PACKAGE"
+NULL
 
 globalVariables(".data")

_pkgdown.yml

@@ -1,3 +1,6 @@
+template:
+  bootstrap: 5
+
 reference:
 - title: Themes
   contents: