docs(calendar): add external controls example (#3793)

Co-authored-by: Benjamin Canac <canacb1@gmail.com>

docs/app/components/content/examples/calendar/CalendarExternalControlsExample.vue

@@ -0,0 +1,21 @@
+<script setup lang="ts">
+import { CalendarDate } from '@internationalized/date'
+
+const date = shallowRef(new CalendarDate(2025, 4, 2))
+</script>
+
+<template>
+  <div class="flex flex-col gap-4">
+    <UCalendar v-model="date" :month-controls="false" :year-controls="false" />
+
+    <div class="flex justify-between gap-4">
+      <UButton color="neutral" variant="outline" @click="date = date.subtract({ months: 1 })">
+        Prev
+      </UButton>
+
+      <UButton color="neutral" variant="outline" @click="date = date.add({ months: 1 })">
+        Next
+      </UButton>
+    </div>
+  </div>
+</template>