ci: install galatea and dev requirements for docs GitHub Action

.github/workflows/documentation.yml

@@ -11,9 +11,9 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
-      - name: Install dependencies
+      - name: Install Galatea with dependencies
         run: |
-          pip install sphinx
+          pip install -r requirements-dev.txt .
       - name: Sphinx build
         run: |
           sphinx-build docs _build