Add CI configuration.

.gitlab-ci.yml

+variables:
+  GIT_SUBMODULE_STRATEGY: recursive
+  TERM: linux
+  GIT_STRATEGY: clone
+
+slides:
+  stage: build
+  only:
+    - master@stephane/ispra-2024
+  script:
+    - make -C tex
+  artifacts:
+    paths:
+      - tex/slides.pdf
+  tags:
+    - stepan@puck
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