docs(react): update component usage docs (#17615)

core/src/components/split-pane/usage/react.md

@@ -0,0 +1,24 @@
+```tsx
+import React from 'react';
+
+import { IonSplitPane, IonMenu, IonHeader, IonToolbar, IonTitle, IonRouterOutlet } from '@ionic/react';
+
+const Example: React.SFC<{}> = () => (
+
+  <IonSplitPane contentId="menuContent">
+    {/*--  our side menu  --*/}
+    <IonMenu contentId="menuContent">
+      <IonHeader>
+        <IonToolbar>
+          <IonTitle>Menu</IonTitle>
+        </IonToolbar>
+      </IonHeader>
+    </IonMenu>
+
+    {/*-- the main content --*/}
+    <IonRouterOutlet></IonRouterOutlet>
+  </IonSplitPane>
+);
+
+export default Example;
+```