Install docs dependencies during readthedocs build

readthedocs.yml

@@ -25,7 +25,7 @@ build:
     post_install:
       # Install dependencies with 'docs' dependency group
       # https://python-poetry.org/docs/managing-dependencies/#dependency-groups
-      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry install
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry install -E docs
       - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry env info
       - pip freeze