.gitlab-ci.yml: Added file to finally host the documentation on gitlab.

[

+pages:
+  script:
+  - cd doc
+  - make html
+  - mv _build/html/* ../public/
+
+  tags:
+  - sphinx
+
+  only:
+  - master
+
+  artifacts:
+    paths:
+    - public
+    expire_in: 5min