Update docs site config for v15 (#429)

- Update config.json
- Update .gitmodules (including ensuring that all whitespace characters
  are consistent)
- Remove the submodule directory for v12
- Add a submodule directory for v16

.gitmodules

@@ -1,16 +1,16 @@
-[submodule "content/12.x"]
-	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 = branch/v13
+	branch = branch/v13
 [submodule "content/14.x"]
 	path = content/14.x
 	url = https://github.com/gravitational/teleport
-        branch = branch/v14
+	branch = branch/v14
 [submodule "content/15.x"]
 	path = content/15.x
 	url = https://github.com/gravitational/teleport
+	branch = branch/v15
+[submodule "content/16.x"]
+	path = content/16.x
+	url = https://github.com/gravitational/teleport
 	branch = master

config.json

@@ -52,19 +52,24 @@
     },
     {
       "name": "12.x",
-      "branch": "branch/v12"
+      "branch": "branch/v12",
+      "deprecated": true
     },
     {
       "name": "13.x",
       "branch": "branch/v13"
     },
     {
       "name": "14.x",
-      "branch": "branch/v14",
-      "latest": true
+      "branch": "branch/v14"
     },
     {
       "name": "15.x",
+      "branch": "branch/v15",
+      "latest": true
+    },
+    {
+      "name": "16.x",
       "branch": "master"
     }
   ]