Update docs to use v12 as default version

.gitmodules

@@ -14,3 +14,7 @@
 	path = content/12.x
 	url = https://github.com/gravitational/teleport
 	branch = branch/v12
+[submodule "content/13.x"]
+	path = content/13.x
+	url = https://github.com/gravitational/teleport
+	branch = master

config.json

@@ -37,19 +37,24 @@
     },
     {
       "name": "9.x",
-      "branch": "branch/v9"
+      "branch": "branch/v9",
+      "deprecated": true
     },
     {
       "name": "10.x",
       "branch": "branch/v10"
     },
     {
       "name": "11.x",
-      "branch": "branch/v11",
-      "latest": true
+      "branch": "branch/v11"
     },
     {
       "name": "12.x",
+      "branch": "branch/v12",
+      "latest": true
+    },
+    {
+      "name": "13.x",
       "branch": "master"
     }
   ]