Add gitlab CI

.gitlab-ci.yml

+image: alpine
+
+workflow:
+  rules:
+    - if: $CI_COMMIT_BRANCH
+
+pages:
+  script:
+  - apk update && apk add doxygen graphviz git
+  - git config --global --add safe.directory $CI_PROJECT_DIR
+  - git submodule update --init doxygen-awesome-css
+  - doxygen Doxyfile
+  - mv doc/html/ public/
+  artifacts:
+    paths:
+    - public
+  rules:
+    - if: $CI_COMMIT_BRANCH == "master"
+  environment: production
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