gitlab-ci: add job to run make check-docs

Add another job to execute `make check-docs`, which lints our
documentation and makes sure that expected manpages exist. This job
mirrors the same job that we already have for GitHub Actions.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

.gitlab-ci.yml

@@ -122,3 +122,12 @@ check-whitespace:
     - ./ci/check-whitespace.sh "$CI_MERGE_REQUEST_TARGET_BRANCH_SHA"
   rules:
     - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+
+documentation:
+  image: ubuntu:latest
+  variables:
+    jobname: Documentation
+  before_script:
+    - ./ci/install-dependencies.sh
+  script:
+    - ./ci/test-documentation.sh