docs: add linkcheck job

This should make it easier to keep links in our docs up-to-date.

But, because links can die behind our backs, we can't really enable this
all over the place, or we'll risk blocking merge-requests due to
unrelated changes.

So let's just make this a periodic job on the main branch instead.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19574>

.gitlab-ci.yml

@@ -183,6 +183,18 @@ test-docs-mr:
       when: on_success
     # Other cases default to never
 
+lincheck-docs:
+  extends: .docs-base
+  # Cancel job if a newer commit is pushed to the same branch
+  interruptible: true
+  stage: deploy
+  needs: []
+  rules:
+    - !reference [.scheduled_pipeline-rules, rules]
+  allow_failure: true
+  variables:
+    BUILDER: linkcheck
+
 # When to automatically run the CI for build jobs
 .build-rules:
   rules: