ci: Add .gitlab-ci.yml triggering the DEEP-SEA CI.

This patch adds a `.gitlab-ci.yml` triggering the downstream pipeline of
the DEEP-SEA CI to automatically run EasyBuild.

.gitlab-ci.yml

+default:
+  tags: [deep-sea, jacamar, slurm ]
+
+variables:
+  SCHEDULER_PARAMETERS: '--account=deepsea --time=03:00:00 --nodes=1 --partition=dp-cn '
+
+stages:
+  - prep
+  - staging
+
+create_tarball:
+  stage: prep
+  script:
+    - module load Autotools
+    - module load Python
+    - echo "Pipeline triggered by and commiting as ${GITLAB_USER_LOGIN}"
+    - echo "Providing tarball on the DEEP system"
+    - mkdir -p -m 777 /p/project/deepsea/ci-stage-sources/p/psmpi
+    - python ./scripts/release.py --suffix=${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-${CI_COMMIT_BRANCH}}
+    - ls -l
+    - cp psmpi-*.tar.gz /p/project/deepsea/ci-stage-sources/p/psmpi/${CI_PIPELINE_ID}.tar.gz
+    - chmod g+rw /p/project/deepsea/ci-stage-sources/p/psmpi/${CI_PIPELINE_ID}.tar.gz
+
+trigger-downstream:
+  stage: staging
+  variables:
+    BUILD_INVERSE_DEPENDENCIES: 'FALSE'
+    EB_FILE_CURRENT: "psmpi-5.9.2-1-GCC-11.3.0.eb"
+    SRC: '${CI_PIPELINE_ID}.tar.gz'
+  trigger:
+    project: deep-sea/wp3/software/easybuild-repository-deep-sea
+    strategy: depend
+    branch: ci-2023-dev
+
+