.gitlab-ci.yml: add gitlab-ci.yml

.gitlab-ci.yml

+pages:
+  variables:
+    DOC_BRANCH: "doc"
+  stage: deploy
+  image: registry.gitlab.com/quarto-forge/docker/polyglot
+  script:
+    - Rscript requirements.R
+    - quarto render ./
+    - mv _site public # gitlab pages should be stored in 'public' directory
+  artifacts:
+    paths:
+      - public
+  rules:
+    - if: $CI_COMMIT_BRANCH == $DOC_BRANCH