chore(doc-gen): convert private classes to interfaces in the docs

Closes #3576

docs/docs-package/processors/convertPrivateClassesToInterfaces.js

@@ -0,0 +1,10 @@
+module.exports = function convertPrivateClassesToInterfacesProcessor(convertPrivateClassesToInterfaces) {
+  return {
+    $runAfter: ['processing-docs'],
+    $runBefore: ['docs-processed'],
+    $process: function(docs) {
+      convertPrivateClassesToInterfaces(docs, false);
+      return docs;
+    }
+  };
+};