Add rehype-mdx-toc to list of plugin

Closes GH-2622.

Reviewed-by: Christian Murphy <[email protected]>
Reviewed-by: Remco Haszing <[email protected]>
Reviewed-by: Titus Wormer <[email protected]>

Author: boning-w

docs/docs/extending-mdx.mdx

@@ -95,6 +95,8 @@ See also the [list of remark plugins][remark-plugins],
   — change media sources to JavaScript imports
 * [`remcohaszing/rehype-mdx-title`](https://github.com/remcohaszing/rehype-mdx-title)
   — 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
 * [`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)