ci: install pandoc

.github/workflows/pages.yml

@@ -11,6 +11,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - name: Install pandoc
+        run: |
+          apt update
+          apt install pandoc -y
       - name: Use Node.js 12.x
         uses: actions/setup-node@v1
         with: