.gitlab-ci.yml: Fix build docs rules

The generic pipelines triggers the build docs jobs based on the
assumption that the docs are in a docs subdirectoy. That is not the case
in the docs repository which includes only documentation.

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

.gitlab-ci.yml

@@ -50,6 +50,11 @@ build-docs:
   script:
     - !reference [.workspace, script]
     - !reference [.build-docs, script]
+  rules:
+    # Build the docs when a merge request is created.
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    # Or when things land.
+    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
 
 deploy:
   extends: .workspace