Temporarily run workflow on PR to test

Author: VisruthSK

.github/workflows/pkgdown.yml

@@ -1,6 +1,7 @@
 # Workflow derived from https://github.com/r-lib/actions/tree/v2/examples
 # Need help debugging build failures? Start at https://github.com/r-lib/actions#where-to-find-help
 on:
+  pull_request:
   # build dev site on merged pushes
   push:
     branches: [main, master]