PNaCl documentation: clarify vector alignment, and update navigation

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

 [
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     "title": "Devtools",
     "items": [
       {
         "title": "Learn Basics",
         "items": [
           {
             "title": "Overview",
             "href": "/devtools/index"
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               },
               {
                 "title": "Native Client Manifest (nmf) Format",
                 "href": "/native-client/reference/nacl-manifest-format"
               },
               {
                 "title": "PNaCl Bitcode Reference Manual",
                 "href": "/native-client/reference/pnacl-bitcode-abi"
               },
               {
+                "title": "PNaCl Undefined Behavior",
+                "href": "/native-client/reference/pnacl-undefined-behavior"
+              },
+              {
                 "title": "PNaCl C/C++ Language Support",
                 "href": "/native-client/reference/pnacl-c-cpp-language-support"
               },
               {
                 "title": "Sandbox Internals",
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                 "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": "Help",
             "href": "/native-client/faq",
             "items": [
               {
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                 "title": "Articles",
                 "href": "/webstore/articles"
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 ]