docs(content-switcher): add reactive example

docs/src/pages/framed/ContentSwitcher/ContentSwitcherReactive.svelte

@@ -0,0 +1,34 @@
+<script>
+  import {
+    ContentSwitcher,
+    Switch,
+    ButtonSet,
+    Button,
+  } from "carbon-components-svelte";
+
+  let selectedIndex = 1;
+</script>
+
+<style>
+  div {
+    margin-top: var(--cds-spacing-05);
+  }
+</style>
+
+<ContentSwitcher bind:selectedIndex>
+  <Switch text="Latest news" />
+  <Switch text="Trending" />
+  <Switch text="Recommended" />
+</ContentSwitcher>
+
+<div>
+  <Button
+    size="small"
+    disabled="{selectedIndex === 2}"
+    on:click="{() => (selectedIndex = 2)}"
+  >
+    Set selected to 2
+  </Button>
+</div>
+
+<div>Selected index: {selectedIndex}</div>