CI: test building the docs

.github/workflows/ci.yml

@@ -55,6 +55,9 @@ jobs:
             - os: ubuntu-20.04
               python-version: '3.9'
               toxenv: mypy
+            - os: ubuntu-20.04
+              python-version: '3.11'
+              toxenv: docs
             - os: ubuntu-20.04
               python-version: '3.9'
               toxenv: py39-fuse2