wip: docs: add build-docs section

Documentation/ci/gitlab-ci/gitlab-ci.rst

@@ -156,6 +156,11 @@ Raises a GitLab warning if "warning" is found in the build log.
    :alt: Matrix of jobs under build-kernel
    :align: center
 
+**job: build-docs**
+
+Builds documentation. Spawns a "sub-job" for each documentation type. Not run
+automatically; requires manual triggering.
+
 stage: cache
 ^^^^^^^^^^^^