Add migration to WebAssembly information to PNaCl / NaCl docs.

chrome/common/extensions/docs/templates/json/chrome_sidenav.json

 [
   {
     "title": "Devtools",
     "items": [
       {
         "title": "Learn Basics",
         "href": "/devtools",
         "items": [
           {
             "title": "Overview",
@@ skipping 704 matching lines @@
                 "title": "Technical Overview",
                 "href": "/native-client/overview"
               },
               {
                 "title": "NaCl and PNaCl",
                 "href": "/native-client/nacl-and-pnacl"
               }
             ]
           },
           {
+            "title": "Migration Guide",

[Ken Rockot(use gerrit already), 2017/05/13 00:27:43]

nit: Should this maybe be "WebAssembly Migration Guide"?

[bradnelson, 2017/05/13 00:35:19]

Done.

+            "href": "/native-client/migration"
+          },
+          {
             "title": "Download the SDK",
             "href": "/native-client/sdk/download",
             "items": [
               {
                 "title": "Download the Native Client SDK",
                 "href": "/native-client/sdk/download"
               },
               {
                 "title": "Examples",
                 "href": "/native-client/sdk/examples"
@@ skipping 318 matching lines @@
                 "title": "Articles",
                 "href": "/webstore/articles"
               }
             ]
           }
         ]
       }
     ]
   }
 ]