Add CI doc building

.gitlab-ci.yml

+stages:
+  - deploy
+
+pages:
+  stage: deploy
+  image: $CI_IMAGE_DOC
+  script:
+    - make doc
+    - mv doc/doxygen/html public
+  artifacts:
+    paths:
+      - public
+  rules:
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH