Add config file types to Project Structure page

We're working on ways to reorganize, or link to, the content in the
"Configuring Astro" guide.

As recommended in
withastro#8918 (comment),
this PR will add the info on supported config file types on the
"Configuring Astro" guide to the "Project Structure" basics page. This
will give new users a quick reference when they're learning how to set
up their projects.

src/content/docs/en/basics/project-structure.mdx

@@ -118,6 +118,10 @@ For help creating a new `package.json` file for your project, check out the [man
 
 This file is generated in every starter template and includes configuration options for your Astro project. Here you can specify integrations to use, build options, server options, and more.
 
+Astro supports several file formats for its JavaScript configuration file: `astro.config.js`, `astro.config.mjs`, `astro.config.cjs` and `astro.config.ts`. We recommend using `.mjs` in most cases or `.ts` if you want to write TypeScript in your config file.
+
+TypeScript config file loading is handled using [`tsm`](https://github.com/lukeed/tsm) and will respect your project tsconfig options.
+
 See the [Configuring Astro Guide](/en/guides/configuring-astro/) for details on setting configurations.
 
 ### `tsconfig.json`