docs: note MDX with remark-gfm usage (vercel#41391)

MDX no longer supports GitHub flavored markdown (GFM) by default since v2, instead requiring a plugin: https://mdxjs.com/guides/gfm/

Since GFM is extremely popular (for example, you need it if you want to use markdown tables), this change paves the cow path, making it clear that next.config.mjs is now supported and allows you to import remark-gfm.

docs/advanced-features/using-mdx.md

@@ -16,7 +16,7 @@ However, because markdown is essentially static content, you can't create _dynam
 
 ## MDX Plugins
 
-Internally MDX uses remark and rehype. Remark is a markdown processor powered by a plugins ecosystem. This plugin ecosystem lets you parse code, transform `HTML` elements, change syntax, extract frontmatter, and more.
+Internally MDX uses remark and rehype. Remark is a markdown processor powered by a plugins ecosystem. This plugin ecosystem lets you parse code, transform `HTML` elements, change syntax, extract frontmatter, and more. Using [remark-gfm to enable GitHub flavored markdown (GFM)](https://mdxjs.com/guides/gfm/) is a popular option.
 
 Rehype is an `HTML` processor, also powered by a plugin ecosystem. Similar to remark, these plugins let you manipulate, sanitize, compile and configure all types of data, elements and content.
 
@@ -44,6 +44,9 @@ The following steps outline how to setup `@next/mdx` in your Next.js project:
    const withMDX = require('@next/mdx')({
      extension: /\.mdx?$/,
      options: {
+       // If you use remark-gfm, you'll need to use next.config.mjs
+       // as the package is ESM only
+       // https://github.com/remarkjs/remark-gfm#install
        remarkPlugins: [],
        rehypePlugins: [],
        // If you use `MDXProvider`, uncomment the following line.