Only build & deploy pkgdown on release

.github/CONTRIBUTING.md

@@ -45,6 +45,16 @@ with write access to code and workflows.
 It should be saved as the `APP_PUSH_TEST_PAT`
 [repository secret](https://github.com/Appsilon/rhino/settings/secrets/actions).
 
+## Website
+
+The [documentation site](https://appsilon.github.io/rhino/)
+is built and deployed automatically by our [`pkgdown.yml`](workflows/pkgdown.yml) workflow.
+This workflow is triggered when a
+[release](https://docs.github.com/en/repositories/releasing-projects-on-github/managing-releases-in-a-repository)
+is published, or a pre-release is changed to a release.
+It is also possible to manually run it for a selected tag/branch
+from the [Actions](https://github.com/Appsilon/rhino/actions/workflows/pkgdown.yml) tab.
+
 ## Release process
 
 ### Preparation

.github/workflows/pkgdown.yml

@@ -1,7 +1,12 @@
 name: pkgdown
 on:
-  push:
-    branches: [main]
+  # Run when a release is published, or a pre-release is changed to a release. Reference:
+  # https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#release
+  # https://docs.github.com/en/webhooks/webhook-events-and-payloads#release
+  release:
+    types: [released]
+  # It is also possible to manually trigger this workflow for a selected tag/branch
+  # from the "Actions" tab on GitHub.
   workflow_dispatch:
 permissions:
   contents: write