add doc for instrumentation client hook

Author: gaojude

docs/01-app/04-api-reference/05-config/01-next-config-js/clientInstrumentationHook.mdx

@@ -0,0 +1,39 @@
+---
+title: clientInstrumentationHook
+description: Enable client-side instrumentation.
+version: experimental
+---
+
+## Usage
+
+When enabled, Next.js searches for an `instrumentation-client.ts` file at the project root and executes it synchronously before hydration. This file is bundled into the initial JavaScript chunk, allowing you to run analytics or monitoring before any other code executes. The code in this file runs exclusively on the client side and is not executed during server-side rendering (SSR).
+
+**Good to know:**
+
+- Client-side instrumentation increases bundle size and execution time, impacting TTI.
+- Running synchronously before hydration, heavy code delays interactivity.
+- Keep it minimal to reduce execution lag and download overhead.
+- Only include what’s essential before hydration.
+
+```ts filename="next.config.ts" switcher
+import type { NextConfig } from 'next'
+
+const nextConfig: NextConfig = {
+  experimental: {
+    clientInstrumentationHook: true,
+  },
+}
+
+export default nextConfig
+```
+
+```js filename="next.config.js" switcher
+/** @type {import('next').NextConfig} */
+const nextConfig = {
+  experimental: {
+    clientInstrumentationHook: true,
+  },
+}
+
+module.exports = nextConfig
+```