Add documentation build step to CI

.gitlab-ci.yml

@@ -43,3 +43,20 @@ build-android:
         - android/**/*
       when: always
     - when: never
+
+build-documentation:
+  image: icaotix/latex:full-incremental
+  stage: build
+  script:
+    - export CI_JOB_TIMESTAMP=$(date --utc -I)
+    - latexmk -pdf documentation
+    - mv "documentation.pdf" "documentation-${CI_JOB_TIMESTAMP}.pdf"
+  artifacts:
+    paths:
+      - "documentation*.pdf"
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
+      changes:
+        - documentation/**/*
+      when: always
+    - when: never