docs(api): add `useRequestEvent` (#5326)

docs/content/3.api/1.composables/use-request-event.md

@@ -0,0 +1,17 @@
+# `useRequestEvent`
+
+Nuxt provides composables and utilities for first-class server-side-rendering support.
+
+Within your pages, components, and plugins you can use `useRequestEvent` to access the incoming request.
+
+```js
+// Get underlying request event
+const event = useRequestEvent()
+
+// Get the URL
+const url = event.req.url
+```
+
+::alert{icon=👉}
+In the browser, `useRequestEvent` will return `undefined`.
+::