add installation of nbsphinx

.gitlab-ci.yml

-
 stages:
   - build
   - test_library
@@ -24,6 +23,7 @@ doc:
     - python -m pip install pip --upgrade
     - python -m pip install setuptools --upgrade
     - python -m pip install sphinx
+    - python -m pip install nbsphinx
     - python -m pip install -r requirements.txt
     - rm -rf artifacts
     - mkdir artifacts