ci: aggiungi concurrency per evitare deploy simultanei

Configurato group "pages" con cancel-in-progress: false.
Ora i deploy si mettono in coda invece di andare in errore.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: aborruso

.github/workflows/jekyll.yml

@@ -9,6 +9,10 @@ on:
       - '.github/workflows/jekyll.yml'
   workflow_dispatch:
 
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
+
 permissions:
   contents: read
   pages: write