build(aio): fix paths to "index" pages

Content pages like `tutorial/index.md` were being mapped to `tutorial.index.json`,
which meant that they could only be rendered if you browsed to `/tutorial/index`.

This didn't sit well so now these pages are mapped to `tutorial.json`, which
means that you browser to them via `/tutorial/` or just `/tutorial`.

Fixed #15335

aio/transforms/angular.io-package/index.js

@@ -266,7 +266,11 @@ module.exports =
               outputPathTemplate: '${moduleDoc.moduleFolder}/${name}.json',
             },
             {docTypes: ['example-region'], getOutputPath: function() {}},
-            {docTypes: ['content'], pathTemplate: '${id}', outputPathTemplate: '${path}.json'},
+            {
+              docTypes: ['content'],
+              getPath: (doc) => `${doc.id.replace(/\/index$/, '')}`,
+              outputPathTemplate: '${path}.json'
+            },
             {docTypes: ['navigation-map'], pathTemplate: '${id}', outputPathTemplate: '../${id}.json'}
           ];
         })