Clarify how MDX plugins are supported in Turbopack (vercel#72917)

Followup to vercel#72241 and
vercel#72467

Documenting the new Turbopack support for MDX plugins (remark, rehype,
recma plugins):

- vercel#71819 (comment)

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### What?

Clarify that MDX-related plugins (remark and rehype plugins) are
supported when options are serializable, by using the new string name
config format

### Why?

Support was added in 72802

- vercel#72802

### How?

Add a new section about using plugins with Turbopack

docs/01-app/02-building-your-application/07-configuring/05-mdx.mdx

@@ -647,9 +647,31 @@ const withMDX = createMDX({
 export default withMDX(nextConfig)
 ```
 
+### Using Plugins with Turbopack
+
+To use plugins with [Turbopack](/docs/architecture/turbopack), upgrade to the latest `@next/mdx` and specify plugin names using a string:
+
+```js filename="next.config.mjs"
+import createMDX from '@next/mdx'
+
+/** @type {import('next').NextConfig} */
+const nextConfig = {
+  pageExtensions: ['js', 'jsx', 'md', 'mdx', 'ts', 'tsx'],
+}
+
+const withMDX = createMDX({
+  options: {
+    remarkPlugins: [],
+    rehypePlugins: [['rehype-katex', { strict: true, throwOnError: true }]],
+  },
+})
+
+export default withMDX(nextConfig)
+```
+
 > **Good to know**:
 >
-> remark and rehype plugins cannot be used with [Turbopack](/docs/architecture/turbopack) yet, due to [inability to pass JavaScript functions to Rust](https://github.com/vercel/next.js/issues/71819#issuecomment-2461802968)
+> remark and rehype plugins without serializable options cannot be used yet with [Turbopack](/docs/architecture/turbopack), due to [inability to pass JavaScript functions to Rust](https://github.com/vercel/next.js/issues/71819#issuecomment-2461802968)
 
 ## Remote MDX