Fixed documentation pipeline job

.gitlab-ci.yml

 stages:
   - test
   - linting
-  - doc
+  - documentation
   - deploy
 
 variables:
@@ -21,7 +21,7 @@ testing:
     - make test_backend_server
   artifacts:
     paths:
-      - htmlcov/
+      - backend/htmlcov/
     expire_in: 1 month
   tags:
     - docker
@@ -44,13 +44,13 @@ eslint:
     - docker
 
 pages:
-  stage: doc
+  stage: documentation
   image: floawfloaw/plantuml
   dependencies:
     - testing
   script:
     - mkdir .public
-    - mv htmlcov/ .public/coverage
+    - mv backend/htmlcov/ .public/coverage
     - make documentation
     - mv documentation/ .public/documentation
     - mv .public public