CI: add documentation job

This job builds all of the Sphinx documentation it can find with fatal
warnings enabled.

Signed-off-by: Ross Burton <ross.burton@arm.com>

.gitlab-ci.yml

@@ -242,3 +242,18 @@ metrics:
   script:
     - kas shell --update --force-checkout ci/base.yml --command \
       "$CI_PROJECT_DIR/ci/patchreview $CI_PROJECT_DIR/meta-* --verbose --metrics $CI_PROJECT_DIR/metrics.txt"
+
+documentation:
+  extends: .setup
+  script:
+    - |
+      sudo pip3 install -r meta-arm-bsp/documentation/requirements.txt
+      for CONF in meta-*/documentation/*/conf.py ; do
+        SOURCE_DIR=$(dirname $CONF)
+        MACHINE=$(basename $SOURCE_DIR)
+        sphinx-build -vW $SOURCE_DIR build-docs/$MACHINE
+      done
+      test -d build-docs/
+  artifacts:
+    paths:
+      - build-docs/