CI: include package installation in MkDocs workflow

The package installation is required by `mkdocs-click`. See mkdocs/mkdocs-click#46 (comment) and mkdocs/mkdocs-click#63.

.github/workflows/material-for-mkdocs.yml

@@ -38,7 +38,7 @@ jobs:
       - name: Install dependencies
         run: |
           poetry config virtualenvs.create false
-          poetry install --only docs
+          poetry install --with docs
 
       - name: Deploy to GitHub Pages
         run: mkdocs gh-deploy --force