Add docs-specific requirements

.github/workflows/ci.yml

@@ -70,6 +70,7 @@ jobs:
         uses: mhausenblas/mkdocs-deploy-gh-pages@master
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          REQUIREMENTS: docs/requirements.txt
 #  deploy:
 #    needs: test
 #    runs-on: ubuntu-latest

docs/requirements.txt

@@ -0,0 +1,3 @@
+-i https://pypi.python.org/simple
+mkdocs==1.3.0
+mkdocs-material==8.2.15