Add remark-directive-mdx to MDX extensions list (#2664)

Closes GH-2664.

Reviewed-by: Remco Haszing <remcohaszing@gmail.com>

Author: re-xyr

docs/docs/extending-mdx.mdx

@@ -97,6 +97,8 @@ See also the [list of remark plugins][remark-plugins],
   — expose the page title as a string
 * [`boning-w/rehype-mdx-toc`](https://github.com/boning-w/rehype-mdx-toc)
   — export the table of contents data into MDX module
+* [`re-xyr/remark-directive-mdx`](https://github.com/re-xyr/remark-directive-mdx)
+  — transform Markdown directives (`:directive[]`) to JSX elements
 * [`pangelani/remark-mdx-chartjs`](https://github.com/pangelani/remark-mdx-chartjs)
   — replace fenced code blocks with charts using [`react-chartjs-2`](https://react-chartjs-2.js.org/).
 * [`remcohaszing/remark-mdx-frontmatter`](https://github.com/remcohaszing/remark-mdx-frontmatter)