Add pandoc setup to GitHub Actions workflow

Author: N400peanuts

.github/workflows/gh-pages.yml

@@ -19,6 +19,10 @@ jobs:
       # 2. Set up R
       - name: Set up R
         uses: r-lib/actions/setup-r@v2
+      
+      # 2.5 Install pandoc
+      - name: Set up pandoc
+        uses: r-lib/setup-pandoc@v2
 
       # 3. Install dependencies
       #