build(aio): move doc-gen stuff from angular.io (#14097)

tools/docs/examples-package/services/getExampleFilename.js

@@ -0,0 +1,9 @@
+module.exports = function getExampleFilename() {
+
+  function getExampleFilenameImpl(relativePath) {
+    return getExampleFilenameImpl.examplesFolder + relativePath;
+  }
+
+  getExampleFilenameImpl.examplesFolder = '@angular/examples/';
+  return getExampleFilenameImpl;
+};