.gitlab-ci.yml: Fix build docs artifacts path

The generic pipelines triggers the build docs jobs based on the
assumption that the documentation build directory is in a docs
subdirectoy. That is not the case in the docs repository which uses the
repository root for the build directory.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

.gitlab-ci.yml

@@ -50,6 +50,9 @@ build-docs:
   script:
     - !reference [.workspace, script]
     - !reference [.build-docs, script]
+  artifacts:
+    paths:
+      - build
   rules:
     # Build the docs when a merge request is created.
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'