Note missing plugins support in mdxRs, refine (#72241)

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### What?

1. Document incompatibility of Turbopack with MDX remark and rehype
plugins
2. Improve casing and prose
3. Add link to `remark-gfm`

### Why?

It's confusing to users that plugins don't work with Turbopack (because
`mdxRs` is required, which [does not support
plugins](https://github.com/wooorm/mdxjs-rs#:~:text=This%20project%20does%20not%20yet%20support%20plugins.)):

- https://bsky.app/profile/matthamlin.bsky.social/post/3la2lzzuprj22

### How?

Edit docs page

Co-authored-by: JJ Kasper <jj@jjsweb.site>

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

@@ -616,21 +616,21 @@ A common use case for this is when you want to iterate over a collection of MDX
 > - Using `fs`, `globby`, etc. can only be used server-side.
 > - View the [Portfolio Starter Kit](https://vercel.com/templates/next.js/portfolio-starter-kit) template for a complete working example.
 
-## Remark and Rehype Plugins
+## remark and rehype Plugins
 
-You can optionally provide `remark` and `rehype` plugins to transform the MDX content.
+You can optionally provide remark and rehype plugins to transform the MDX content.
 
-For example, you can use `remark-gfm` to support GitHub Flavored Markdown.
+For example, you can use [`remark-gfm`](https://github.com/remarkjs/remark-gfm) to support GitHub Flavored Markdown.
 
-Since the `remark` and `rehype` ecosystem is ESM only, you'll need to use `next.config.mjs` as the configuration file.
+Since the remark and rehype ecosystem is ESM only, you'll need to use `next.config.mjs` or `next.config.ts` as the configuration file.
 
 ```js filename="next.config.mjs"
 import remarkGfm from 'remark-gfm'
 import createMDX from '@next/mdx'
 
 /** @type {import('next').NextConfig} */
 const nextConfig = {
-  // Configure `pageExtensions`` to include MDX files
+  // Allow .mdx extensions for files
   pageExtensions: ['js', 'jsx', 'md', 'mdx', 'ts', 'tsx'],
   // Optionally, add any other Next.js config below
 }
@@ -643,10 +643,14 @@ const withMDX = createMDX({
   },
 })
 
-// Wrap MDX and Next.js config with each other
+// Combine MDX and Next.js config
 export default withMDX(nextConfig)
 ```
 
+> **Good to know**:
+>
+> remark and rehype plugins cannot be used with [Turbopack](/docs/architecture/turbopack), due to lack of plugins support in [`mdxRs`](#using-the-rust-based-mdx-compiler-experimental)
+
 ## Remote MDX
 
 If your MDX files or content lives _somewhere else_, you can fetch it dynamically on the server. This is useful for content stored in a separate local folder, CMS, database, or anywhere else. A popular community package for this use is [`next-mdx-remote`](https://github.com/hashicorp/next-mdx-remote#react-server-components-rsc--nextjs-app-directory-support).