Ajout d'un .gitlab-ci.yml pour la génération automatique de doc

.gitlab-ci.yml

+image: alpine
+
+pages:
+  script:
+  - apk update && apk add doxygen graphviz ttf-freefont
+  - doxygen docs/doxygen.conf
+  - mv docs/html/ public/
+  artifacts:
+    paths:
+    - public
+  only:
+  - master
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