Add Pandoc installation step to CI workflow for documentation build: required to convert notebook markdown cells into reStructuredText or HTML

Author: bmusangu

.github/workflows/tests.yml

@@ -46,6 +46,11 @@ jobs:
         with:
           python-version: '3.x'
 
+      - name: Install Pandoc
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y pandoc
+
       - name: Install docs dependencies
         run: |
           python -m pip install --upgrade pip