Setup for multi-project pipelines

Since we should now be able to use multi-project pipelines, setup the
main branch so that we can use it in a multi-project pipeline.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

.gitlab-ci.yml

@@ -9,3 +9,8 @@ archive-tools:
       - atc_extra
       - atc_menu
       - static
+
+pages:
+  extends: .deploy-pages
+  only:
+    - main