NaCl docs: Add a list of past design docs

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

 [
   {
     "title": "Devtools",
     "items": [
       {
         "title": "Learn Basics",
         "items": [
           {
             "title": "Overview",
             "href": "/devtools/index"
@@ skipping 863 matching lines @@
                 "items": [
                     {
                       "title": "ARM 32-bit Sandbox",
                       "href": "/native-client/reference/sandbox_internals/arm-32-bit-sandbox"
                     },
                     {
                       "title": "x86-64 Sandbox",
                       "href": "/native-client/reference/sandbox_internals/x86-64-sandbox"
                     }
                 ]
+              },
+              {
+                "title": "Design Documents",
+                "href": "/native-client/reference/design-docs"
               }
             ]
           },
           {
             "title": "Help",
             "href": "/native-client/faq",
             "items": [
               {
                 "title": "FAQ",
                 "href": "/native-client/faq"
@@ skipping 123 matching lines @@
                 "title": "Articles",
                 "href": "/webstore/articles"
               }
             ]
           }
         ]
       }
     ]
   }
 ]