docs: add parallel code splitting docs (#9051)

Author: JSerFeng

website/docs/en/config/experiments.mdx

@@ -307,6 +307,23 @@ Currently, incremental for the `make` and `emitAssets` stages is enabled by defa
 This feature is experimental. You can check its relevant progress at [rspack#8106](https://github.com/web-infra-dev/rspack/issues/8106). You can also report bugs and any related feedback here.
 :::
 
+## experiments.parallelCodeSplitting
+
+<ApiMeta addedVersion="1.2.0" />
+
+- **Type:** `boolean`
+- **Default:** `false`
+
+Enabling this configuration will activate a new multi-threaded code splitting algorithm. If your project includes many dynamic imports, this can greatly reduce the time spent on the code splitting process. In the future, Rspack will have this enabled by default.
+
+```js title="rspack.config.js"
+module.exports = {
+  experiments: {
+    parallelCodeSplitting: true,
+  },
+};
+```
+
 ## experiments.rspackFuture
 
 - **Type:** `object`

website/docs/zh/config/experiments.mdx

@@ -305,6 +305,23 @@ module.exports = {
 该特性属于实验性特性，可前往 [rspack#8106](https://github.com/web-infra-dev/rspack/issues/8106) 查看其相关进度，也可在此 issue 中反馈 bugs 以及任何与之相关的反馈信息。
 :::
 
+## experiments.parallelCodeSplitting
+
+<ApiMeta addedVersion="1.2.0" />
+
+- **类型：** `boolean`
+- **默认值：** `false`
+
+开启后会启用新的多线程 code splitting 算法，如果你的项目中包含较多的动态引用，开启后可以显著降低 code splitting 阶段耗时，在未来 Rspack 会默认开启这个配置项。
+
+```js title="rspack.config.js"
+module.exports = {
+  experiments: {
+    parallelCodeSplitting: true,
+  },
+};
+```
+
 ## experiments.rspackFuture
 
 - **类型：** `object`