Test landingpage example build in Github actions

Author: mtancoigne

.github/workflows/actions.yml

@@ -32,3 +32,8 @@ jobs:
       run: yarn build:lib
     - name: Build documentation
       run: yarn build
+    - name: Build examples
+      run: 
+        - cd _examples/landingpage
+        - yarn install
+        - yarn build