docs: add advanced usage example of useState (#20249)

docs/1.getting-started/7.state-management.md

@@ -59,10 +59,71 @@ const counter = useState('counter', () => Math.round(Math.random() * 1000))
 ::ReadMore{link="/docs/api/composables/use-state"}
 ::
 
-### Advanced
+### Advanced Usage
 
 In this example, we use a composable that detects the user's default locale from the HTTP request headers and keeps it in a `locale` state.
 
+```ts [composables/locale.ts]
+import type { Ref } from 'vue'
+
+export const useLocale = () => useState<string>('locale', () => useDefaultLocale().value)
+
+export const useDefaultLocale = (fallback = 'en-US') => {
+  const locale = ref(fallback)
+  if (process.server) {    
+    const reqLocale = useRequestHeaders()['accept-language']?.split(',')[0]
+    if (reqLocale) {
+      locale.value = reqLocale
+    }
+  } else if (process.client) {
+    const navLang = navigator.language
+    if (navLang) {
+      locale.value = navLang
+    }
+  }
+  return locale
+}
+
+export const useLocales = () => {
+  const locale = useLocale()
+  const locales = ref([
+    'en-US',
+    'en-GB',
+    ...
+    'ja-JP-u-ca-japanese'
+  ])
+  if (!locales.value.includes(locale.value)) {
+    locales.value.unshift(locale.value)
+  }
+  return locales
+}
+
+export const useLocaleDate = (date: Ref<Date> | Date, locale = useLocale()) => {
+  return computed(() => new Intl.DateTimeFormat(locale.value, { dateStyle: 'full' }).format(unref(date)))
+}
+```
+
+```vue [app.vue]
+<script setup>
+const locales = useLocales()
+const locale = useLocale()
+const date = useLocaleDate(new Date('2016-10-26'))
+</script>
+
+<template>
+  <div>
+    <h1>Nuxt birthday</h1>
+    <p>{{ date }}</p>    
+    <label for="locale-chooser">Preview a different locale</label>
+    <select id="locale-chooser" v-model="locale">
+      <option v-for="locale of locales" :key="locale" :value="locale">
+        {{ locale }}
+      </option>
+    </select>
+  </div>
+</template>
+```
+
 ::LinkExample{link="/docs/examples/other/locale"}
 ::