[ci] Added build docs job.

scripts/ci/jobs/build.yml

+build docs:
+  stage: test
+  image:
+    name: $CONTAINER_GCC_IMAGE
+    entrypoint:
+      - '/usr/bin/env'
+      - 'PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
+
+  before_script:
+    - mkdir -p build
+    - cd build
+
+  script:
+    - cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Release -DOGS_USE_CONAN=OFF -DOGS_BUILD_PROCESSES=GroundwaterFlow
+    - cmake --build . --target doc > >(tee make-docs.output)
+
+  artifacts:
+    expose_as: 'Doxygen'
+    paths: ['build/docs/']
+
 build:
   stage: test
   variables: