docs: add example of useRuntimeConfig() within a plugin (#2900)

docs/content/3.docs/1.usage/5.runtime-config.md

@@ -77,6 +77,21 @@ const config = useRuntimeConfig()
 **`useRuntimeConfig` only works during `setup` or `Lifecycle Hooks`**.
 ::
 
+### Plugins
+
+If you want to use the runtime confg within any (custom) plugin, you can use `useRuntimeConfig()` inside of your `defineNuxtPlugin` function.
+
+For example:
+```ts
+export default defineNuxtPlugin((nuxtApp) => {
+  const config = useRuntimeConfig();
+
+  const url = process.server ? config.serverUrl : config.clientUrl;
+  
+  // Do something with url & isServer.
+});
+```
+
 ### API routes
 
 Within the API routes, you can access runtime config by directly importing from virtual `#config`.