[gtkmm-documentation] CI: Fix the deploy stage

[Kjell Ahlstedt <kjellahl src gnome org>]

commit ebd9935b6f5395f44b20952128a6d2414e7f490b
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Aug 18 13:42:43 2021 +0200

    CI: Fix the deploy stage
    
    See !13

 .gitlab-ci.yml | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 5af7744..20ba2d7 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -56,6 +56,9 @@ pages:
   script:
     - mkdir public
     - mv _build/docs/tutorial public
+  artifacts:
+    paths:
+      - public
   only:
     refs:
       - master