Add concurrency control to gh-pages deployment

Prevents race conditions when multiple CI runs try to deploy
simultaneously by queuing deployments instead of running in parallel.

Author: cs01

.github/workflows/deploy-playground.yml

@@ -9,6 +9,10 @@ on:
 permissions:
   contents: write
 
+concurrency:
+  group: deploy-gh-pages
+  cancel-in-progress: false
+
 jobs:
   deploy:
     runs-on: ubuntu-latest