chore(ci): use poetry to install docs

.github/workflows/docs.yml

@@ -22,8 +22,8 @@ jobs:
       - name: Install dependencies
         id: install
         run: |
-          python -m pip install --upgrade pip
-          pip install mkdocs mkdocstrings mkdocs-material
+          pipx install poetry --python $(which python)
+          poetry install --only docs
       - name: Build docs
         id: build
         run: |