collapsible panel redesign (#487)

src/theme/MDXComponents.js

@@ -0,0 +1,20 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+// Import the original mapper
+import MDXComponents from '@theme-original/MDXComponents';
+import Collapsible from '../components/Collapsible';
+
+export default {
+  // Re-use the default mapping
+  ...MDXComponents,
+  // Map the "collapsible" tag to our <Collapsible /> component!
+  // `Collapsible` will receive all props that were passed to `collapsible` in MDX
+  Collapsible,
+};