docs(VerticalNavigation): add slots examples (#456)

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

docs/components/content/examples/VerticalNavigationExampleAvatarSlot.vue

@@ -0,0 +1,25 @@
+<script setup>
+const links = [{
+  avatar: {
+    src: 'https://avatars.githubusercontent.com/u/739984?v=4'
+  },
+  label: 'Benjamin Canac'
+}, {
+  label: 'KeJun'
+}]
+
+const { ui } = useAppConfig()
+</script>
+
+<template>
+  <UVerticalNavigation :links="links">
+    <template #avatar="{ link }">
+      <UAvatar
+        v-if="link.avatar"
+        v-bind="{ size: ui.verticalNavigation.avatar.size, ...link.avatar }"
+        :class="[ui.verticalNavigation.avatar.base]"
+      />
+      <UIcon v-else name="i-heroicons-user-circle-20-solid" class="text-lg" />
+    </template>
+  </UVerticalNavigation>
+</template>