Adding banner on mainpage to warn about SSO for D9 websites

docs/docs/1-Getting Started/1-introduction.md

@@ -2,6 +2,13 @@
 slug: /
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+:::danger Upgrade to Drupal 10 with CKEditor 4
+
+Drupal 9 websites will be put behind SSO on 18th of December if they have not contacted support.   
+Details in [OTG0145045](https://cern.service-now.com/service-portal?id=outage&n=OTG0145045)
+
+:::
+
 # Introduction
 
 Welcome to **drupal.docs.cern.ch**, your one-stop Drupal resource at CERN.