Fix documentation workflow

.github/workflows/documentation.yml

@@ -19,13 +19,31 @@ jobs:
       with:
         python-version: 3.9
 
+    - name: Install poetry
+      uses: snok/install-poetry@v1
+      with:
+        virtualenvs-create: true
+        virtualenvs-in-project: true
+
+    - name: Load cached venv
+      id: cached-poetry-dependencies
+      uses: actions/cache@v3
+      with:
+        path: .venv
+        key: venv-${{ runner.os }}-${{ hashFiles('**/poetry.lock') }}
+
     - name: Install dependencies
+      if: steps.cached-poetry-dependencies.outputs.cache-hit != 'true'
+      run: |
+        poetry install --no-interaction --no-root
+
+    - name: Install library
       run: |
-        python -m pip install --upgrade pip
-        pip install Sphinx sphinx-rtd-theme sphinx-mdinclude
+        poetry install --no-interaction
 
     - name: Generate documentation with Sphinx
       run: |
+        source .venv/bin/activate
         make docs
         touch docs/_build/html/.nojekyll