docs(slideover): add close button in some examples for mobile (#1827)

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

docs/components/content/examples/SlideoverExampleBasic.vue

@@ -8,6 +8,16 @@ const isOpen = ref(false)
 
     <USlideover v-model="isOpen">
       <div class="p-4 flex-1">
+        <UButton
+          color="gray"
+          variant="ghost"
+          size="sm"
+          icon="i-heroicons-x-mark-20-solid"
+          class="flex sm:hidden absolute end-5 top-5 z-10"
+          square
+          padded
+          @click="isOpen = false"
+        />
         <Placeholder class="h-full" />
       </div>
     </USlideover>