Migration gitlab doc

We publish GATB-CORE API's doxygen documentation on the project's gitlabpages.

See .gitlab-ci.yml on the deploy-api-doc branch. This pipeline is triggered by the https://ci.inria.fr/gatb-core/view/GATB-CORE-gitlab/job/publish-doc-api-gitlab/ Jenkins task.

The Jenkins task prepares the doxygen documentation (as it already used to), and makes a tarfile artifact available. The gitlab-ci pipeline then downloads this Jenkins artifact, and deploys it on the project's gitlabpages.

From now on, the API documentation will be available here: https://gatb.gitlabpages.inria.fr/gatb-core/doc/api/index.html