Update gh-pages workflow

.github/workflows/gh-pages.yaml

@@ -21,7 +21,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -r requirements_dev.txt && pip install sphinxcontrib-googleanalytics
+          pip install -r requirements_dev.txt && pip install sphinxcontrib-googleanalytics sphinx-copybutton
       - name: Build Sphinx docs
         run: |
           make docs