docs(plugins): add source map introduction for transform hook (#6052)

Author: chenjiahan

website/docs/en/plugins/dev/core.mdx

@@ -511,6 +511,20 @@ In Rsbuild plugins, you can quickly implement code transformation functions usin
 
 Note that for some complex code transformation scenarios, `api.transform` may not be sufficient. In such situations, you can implement it using the Rspack loader.
 
+### Source maps
+
+You can return a source map in the `transform` function, and Rsbuild will automatically merge the returned source map with source maps generated by other Rspack loaders or `transform` hooks, ensuring that the final source map correctly maps back to the original source code.
+
+```js
+api.transform({ test: /\.js$/ }, async ({ code }) => {
+  const { transformedCode, sourceMap } = await someTransformFunction(code);
+  return {
+    code: transformedCode,
+    map: sourceMap,
+  };
+});
+```
+
 ## api.resolve
 
 Intercept and modify module request information before module resolution begins. The same as Rspack's [normalModuleFactory.hooks.resolve](https://rspack.rs/api/plugin-api/normal-module-factory-hooks#resolve) hook.

website/docs/zh/plugins/dev/core.mdx

@@ -511,6 +511,20 @@ api.transform(
 
 注意，对于一些复杂的代码转换场景，`api.transform` 可能无法满足，此时你可以使用 Rspack loader 进行实现。
 
+### Source maps
+
+你可以在 `transform` 函数中返回一个 source map，Rsbuild 会自动将返回的 source map 与其他 Rspack loader 或 `transform` hook 生成的 source maps 合并，使最终的 source map 能够正确映射到原始源代码。
+
+```js
+api.transform({ test: /\.js$/ }, async ({ code }) => {
+  const { transformedCode, sourceMap } = await someTransformFunction(code);
+  return {
+    code: transformedCode,
+    map: sourceMap,
+  };
+});
+```
+
 ## api.resolve
 
 在模块解析开始之前，拦截并修改模块的请求信息。等价于 Rspack 的 [normalModuleFactory.hooks.resolve](https://rspack.rs/zh/api/plugin-api/normal-module-factory-hooks#resolve) hook。