[DOCS] Add live branches to ECE docs (#2748)

Adds a `live` branch property to the ECE docs. This ensures that branches for 2.x docs are no longer indexed.

Author: jrodewig

conf.yaml

@@ -802,6 +802,14 @@ contents:
             tags:       CloudEnterprise/Reference
             subject:    ECE
             current:    ms-92
+            live:
+              - ms-92
+              - ms-81
+              - ms-78
+              - ms-75
+              - ms-72
+              - ms-70
+              - ms-69
             branches:
               - ms-92: 3.6
               - ms-81: 3.5