Add docs branch for cord-7.0 release

Change-Id: Ibe6c79b84d62d811867bf4890601aa3f620dec3f
diff --git a/book.json b/book.json
index f832c23..b1db26d 100644
--- a/book.json
+++ b/book.json
@@ -9,7 +9,7 @@
     "website": "styles/website.css"
   },
   "variables": {
-    "branch": "master"
+    "branch": "cord-7.0"
   },
   "plugins": [
     "anchorjs",
@@ -32,6 +32,10 @@
           "text": "Master (Devel)"
         },
         {
+          "value": "/cord-7.0",
+          "text": "7.0 (Stable)"
+        },
+        {
           "value": "/cord-6.1",
           "text": "6.1 (Stable)"
         },