docs(SelectMenu): add slots examples

Resolves #557

docs/components/content/examples/SelectMenuExampleOptionEmptySlot.vue

@@ -0,0 +1,13 @@
+<script setup>
+const people = ['Wade Cooper', 'Arlene Mccoy', 'Devon Webb', 'Tom Cook', 'Tanya Fox', 'Hellen Schmidt', 'Caroline Schultz', 'Mason Heaney', 'Claudie Smitham', 'Emil Schaefer']
+
+const selected = ref(people[0])
+</script>
+
+<template>
+  <USelectMenu v-model="selected" :options="people" searchable>
+    <template #option-empty="{ query }">
+      <q>{{ query }}</q> not found
+    </template>
+  </USelectMenu>
+</template>