docs: add PostCSS/Tailwind support to v2 guide (#6910)

docs/pages/v2.md

@@ -862,6 +862,19 @@ remix dev --manual -c 'node ./server.js'
 
 Check out the [manual mode guide][manual-mode] for more details.
 
+## Built-in PostCSS/Tailwind support
+
+In v2, these tools will be automatically used within the Remix compiler if PostCSS and/or Tailwind configuration files are present in your project.
+
+If you have a custom PostCSS and/or Tailwind setup outside of Remix that you'd like to maintain when migrating to v2, you can disable these features in your `remix.config.js`.
+
+```js filename=remix.config.js
+module.exports = {
+  postcss: false,
+  tailwind: false,
+};
+```
+
 [future-flags]: ./api-development-strategy
 [remix-config]: ../file-conventions/remix-config
 [flat-routes]: https://github.com/remix-run/remix/discussions/4482