feat(v2): implement theme component overriding (#1435)

* feat(v2): implement component overriding

* siteDir theme overriding should work for > 1 level directory

* fallback for essential component like Loading

* rename default -> classic

packages/docusaurus-theme-classic/src/index.js

@@ -0,0 +1,36 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const path = require('path');
+
+const DEFAULT_OPTIONS = {};
+
+class DocusaurusThemeDefault {
+  constructor(context, opts) {
+    this.options = {...DEFAULT_OPTIONS, ...opts};
+    this.context = context;
+  }
+
+  getName() {
+    return 'docusaurus-theme-classic';
+  }
+
+  configureWebpack() {
+    return {
+      resolve: {
+        alias: {
+          '@theme/Footer': path.resolve(__dirname, './theme/Footer'),
+          '@theme/Navbar': path.resolve(__dirname, './theme/Navbar'),
+          '@theme/NotFound': path.resolve(__dirname, './theme/NotFound'),
+          '@theme/Search': path.resolve(__dirname, './theme/Search'),
+        },
+      },
+    };
+  }
+}
+
+module.exports = DocusaurusThemeDefault;