Update pip to latest version at start of deploy flow

Currently, pip 22.0.2 is being used, which is broken:
pypa/pip#10851

upgrading to the latest version makes our docs deployment work again

.github/workflows/docs.yml

@@ -11,6 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
+      - run: python -m pip install --upgrade pip
       - run: pip install mkdocs mkdocs-material
       - run: cd docs
       - run: mkdocs build -f mkdocs.yml