docs: add note regarding ts extension for config file. (#10082)

website/docs/api/docusaurus.config.js.mdx

@@ -18,6 +18,12 @@ Refer to the Getting Started [**Configuration**](../configuration.mdx) for examp
 
 `docusaurus.config.js` contains configurations for your site and is placed in the root directory of your site.
 
+:::note
+
+With a [TypeScript](../typescript-support.mdx) Docusaurus codebase your config file may be called `docusaurus.config.ts`. The syntax is broadly identical to the `js` config file with the addition of types. You can see an example on the [Docusaurus Website](https://github.com/facebook/docusaurus/blob/main/website/docusaurus.config.ts) itself.
+
+:::
+
 This file is run in Node.js and should export a site configuration object, or a function that creates it.
 
 The `docusaurus.config.js` file supports: