Add remark-mdx-chartjs to list of plugins

Closes GH-2221.

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

Author: pangelani

docs/docs/extending-mdx.server.mdx

@@ -81,6 +81,8 @@ See also the [list of remark plugins][remark-plugins] and
     — change frontmatter (YAML) metadata to exports
 *   [`goodproblems/remark-mdx-math-enhanced`](https://github.com/goodproblems/remark-mdx-math-enhanced)
     — enhance math with JavaScript inside it
+*   [`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/).
 
 {/*
 please use alpha sorting on **project** name!