docs: webpack-merge also applies to rspack (web-infra-dev#7953)

* docs: webpack-merge also applies to rspack

* docs: merge configurations

website/docs/en/config/index.mdx

@@ -142,3 +142,21 @@ module.exports = function (env, argv) {
   };
 };
 ```
+
+## Merge Configurations
+
+You can use the `merge` function exported by `webpack-merge` to merge multiple configurations.
+
+```js title="rspack.config.js"
+const { merge } = require('webpack-merge');
+
+const base = {};
+
+const dev = {
+  plugins: [new DevelopmentSpecifiedPlugin()]
+};
+
+module.exports = process.env.NODE_ENV === 'development' ? merge(base, dev) : base;
+```
+
+For more information of `merge`, please refer to [webpack-merge documentation](https://npmjs.com/package/webpack-merge).

website/docs/zh/config/index.mdx

@@ -140,3 +140,21 @@ module.exports = function (env, argv) {
   };
 };
 ```
+
+## 合并配置
+
+你可以使用 `webpack-merge` 导出的 `merge` 函数来合并多个配置。
+
+```js title="rspack.config.js"
+const { merge } = require('webpack-merge');
+
+const base = {};
+
+const dev = {
+  plugins: [new DevelopmentSpecifiedPlugin()]
+};
+
+module.exports = process.env.NODE_ENV === 'development' ? merge(base, dev) : base;
+```
+
+关于 `merge` 的更多信息请查看 [webpack-merge 文档](https://npmjs.com/package/webpack-merge)。