ci: Don't create test-docs job if the pages one exists in the pipeline

It's redundant in that case.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6566>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 15eb5c8..119d99d 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -100,6 +100,9 @@
   stage: container+docs
   rules:
     - *ignore_scheduled_pipelines
+    - if: *is-main-master
+      changes: *docs-or-ci
+      when: never
     - if: *is-pre-merge-for-marge
       changes: *docs-or-ci
       when: on_success