Freeze docs checkout to specific commits
Update book.json to include 6.0 in version selector

Change-Id: Ie34d4c603100ed9391929d8ceffbe3d54cff049b
diff --git a/book.json b/book.json
index fd3f609..1317bf7 100644
--- a/book.json
+++ b/book.json
@@ -21,6 +21,10 @@
           "text": "Master (Devel)"
         },
         {
+          "value": "/cord-6.0",
+          "text": "6.0 (Stable)"
+        },
+        {
           "value": "/cord-5.0",
           "text": "5.0 (Stable)"
         },
@@ -29,10 +33,6 @@
           "text": "4.1 (Stable)"
         },
         {
-          "value": "/cord-4.0",
-          "text": "4.0 (Stable)"
-        },
-        {
           "value": "https://wiki.opencord.org/display/CORD/Building+and+Installing+CORD",
           "text": "3.0 and previous (wiki)"
         }