Added React Strict Mode documentation (vercel#13723)

Added new documentation for React Strict mode under the `next.config.js` doc level.

Closes vercel#13713

docs/api-reference/next.config.js/react-strict-mode.md

@@ -0,0 +1,29 @@
+---
+description: The complete Next.js runtime is now Strict Mode-compliant, learn how to opt-in
+---
+
+# React Strict Mode
+
+> **Suggested**: We strongly suggest you enable Strict Mode in your Next.js application to better prepare your application for the future of React.
+
+The Next.js runtime is now Strict Mode-compliant. To opt-in to Strict Mode, configure the following option in your `next.config.js`:
+
+```js
+// next.config.js
+module.exports = {
+  reactStrictMode: true,
+}
+```
+
+If you or your team are not ready to use Strict Mode in your entire application, that's OK! You can incrementally migrate on a page-by-page basis [using `<React.StrictMode>`](https://reactjs.org/docs/strict-mode.html).
+
+React's Strict Mode is a development mode only feature for highlighting potential problems in an application. It helps to identify unsafe lifecycles, legacy API usage, and a number of other features.
+
+## Related
+
+<div class="card">
+  <a href="/docs/api-reference/next.config.js/introduction.md">
+    <b>Introduction to next.config.js:</b>
+    <small>Learn more about the configuration file used by Next.js.</small>
+  </a>
+</div>

docs/manifest.json

@@ -274,6 +274,10 @@
             {
               "title": "exportPathMap",
               "path": "/docs/api-reference/next.config.js/exportPathMap.md"
+            },
+            {
+              "title": "React Strict Mode",
+              "path": "/docs/api-reference/next.config.js/react-strict-mode.md"
             }
           ]
         }