Update .gitlab-ci.yml: failing pages seems to be related to new sphinx version

.gitlab-ci.yml

@@ -41,6 +41,7 @@ docs:
   script:
     - zypper --non-interactive install python3-pip
     - pip install --upgrade pip
+    - pip install --upgrade sphinx==3.4.3
     - pip install --no-cache-dir -r requirements.txt
     - pip install --no-cache-dir -r requirements_for_documentation.txt
     - chmod +x ./CI/create_documentation.sh