Adds caching for pip dependencies.

.github/workflows/sphinx-gh-pages.yml

@@ -36,6 +36,10 @@ jobs:
         uses: actions/checkout@v4
       - name: Setup Pages
         uses: actions/configure-pages@v5
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.10'
+          cache: 'pip'
       - name: Setup sphinx
         run: |
           pip install -r documentation/requirements.txt