Add `AttachmentSection` to style guide

ts/components/conversation/media-gallery/AttachmentSection.md

@@ -0,0 +1,24 @@
+```jsx
+const messages = [
+  {
+    attachments: [
+      {
+        fileName: 'foo.json',
+        contentType: 'application/json',
+        size: 53313,
+      },
+    ],
+  },
+  {
+    attachments: [
+      {
+        fileName: 'bar.txt',
+        contentType: 'text/plain',
+        size: 10323,
+      },
+    ],
+  },
+];
+
+<AttachmentSection header="Today" type="documents" messages={messages} />;
+```