feat: rewrite to use app config and rework docs (#143)

Co-authored-by: Daniel Roe <daniel@roe.dev>
Co-authored-by: Sébastien Chopin <seb@nuxt.com>

docs/components/content/examples/NotificationExampleCallback.vue

@@ -0,0 +1,11 @@
+<script setup>
+const toast = useToast()
+
+function onCallback () {
+  alert('Notification expired!')
+}
+</script>
+
+<template>
+  <UButton label="Show toast" @click="toast.add({ title: 'Expires soon...', timeout: 1000, callback: onCallback })" />
+</template>