docs/ci: allow sanity job to be missing

The rules of both jobs never matched, so there was always the potential
for the job to be missing, but it's about to get worse with the next
commit, so fix that now.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25920>

docs/gitlab-ci.yml

@@ -47,7 +47,8 @@ test-docs-mr:
   extends:
     - test-docs
   needs:
-    - sanity
+    - job: sanity
+      optional: true
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
       changes: *docs-or-ci