Install cmdstanR in GHA before building site

Author: VisruthSK

.github/workflows/pkgdown.yaml

@@ -36,6 +36,9 @@ jobs:
         with:
           extra-packages: any::pkgdown, local::., stan-dev/pkgdown-config
 
+      - name: Install CmdStan
+        run: Rscript -e "cmdstanr::install_cmdstan()"
+      
       - name: Build site
         run: |
           pkgdown::build_site_github_pages(