[Docs Site] Only run publish-preview on original repository (#20382)

Author: KianNH

.github/workflows/publish-preview.yml

@@ -9,6 +9,7 @@ concurrency:
 
 jobs:
   publish:
+    if: github.repository == 'cloudflare/cloudflare-docs'
     runs-on: ubuntu-22.04
     permissions:
       contents: read