added artifacts

.gitlab-ci.yml

@@ -8,6 +8,9 @@ build:
     - cd docs
   script:
     -  nix-shell -p python3Packages.sphinx python3Packages.numpydoc python3Packages.pnam --run "make html"
+  artifacts:
+    paths:
+      - docs/_build/
 
 deploy:
   stage: deploy