ci: Don't deploy docs on forks

It would fail unless the fork also has docs build artefacts for all
previous releases.

Based on https://gitlab.gnome.org/World/Rust/gir-rustdoc/-/merge_requests/10

.gitlab-ci.yml

@@ -33,6 +33,7 @@ variables:
   # the name is used in the URL
   # latest release must be at the top
   # (only relevant on master branch)
+  NAMESPACE: gstreamer
   RELEASES:
     0.17=0.17
     0.16=0.16
@@ -498,5 +499,5 @@ pages:
     paths:
       - 'public'
   rules:
-    - if: $CI_COMMIT_BRANCH == "master" || $CI_COMMIT_BRANCH == "main"
+    - if: $CI_DEFAULT_BRANCH == $CI_COMMIT_BRANCH && $CI_PROJECT_NAMESPACE == $NAMESPACE
       when: 'manual'