NaCl docs: add project ideas

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

 [
   {
     "title": "Devtools",
     "items": [
       {
         "title": "Learn Basics",
         "href": "/devtools",
         "items": [
           {
             "title": "Overview",
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           },
           {
             "title": "Additional Reference & Versions",
             "href": "/native-client/glossary",
             "items": [
               {
                 "title": "Glossary",
                 "href": "/native-client/glossary"
               },
               {
+                "title": "Contributor Ideas",
+                "href": "/native-client/reference/ideas"
+              },
+              {
                 "title": "Native Client Manifest (nmf) Format",
                 "href": "/native-client/reference/nacl-manifest-format"
               },
               {
                 "title": "Contents of PNaCl Bitcode Files",
                 "href": "/native-client/reference/pnacl-bitcode-manual"
               },
               {
                 "title": "PNaCl Bitcode Reference Manual",
                 "href": "/native-client/reference/pnacl-bitcode-abi"
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                 "title": "Articles",
                 "href": "/webstore/articles"
               }
             ]
           }
         ]
       }
     ]
   }
 ]