Use the dev version of pkgdown (#4543)

* Use latest pkgdown

* Add a missing topic

Author: yutannihilation

.github/workflows/pkgdown.yaml

@@ -47,7 +47,7 @@ jobs:
       - name: Install dependencies
         run: |
           pak::local_install_dev_deps(upgrade = TRUE, dependencies = c("all", "Config/Needs/website"))
-          pak::pkg_install("pkgdown")
+          pak::pkg_install("r-lib/pkgdown")
         shell: Rscript {0}
 
       - name: Install package

_pkgdown.yml

@@ -118,6 +118,7 @@ reference:
   - expand_limits
   - expansion
   - starts_with("scale_")
+  - get_alt_text
 
 - title: "Guides: axes and legends"
   desc: >