Mention that node_modules is ignored by default (#2298)

As of v4.1.0, Tailwind ignores the `node_modules` directory by default,
regardless of your `.gitignore` settings.

*
https://github.com/tailwindlabs/tailwindcss/blob/main/CHANGELOG.md#410---2025-04-01

> Ignore `node_modules` by default (can be overridden by `@source` …
rules) (tailwindlabs/tailwindcss#17255)

This is worth mentioning in the relevant sections of the documentation:

* [Which files are
scanned](https://tailwindcss-com-git-fork-rozsazoltan-update-604522-tailwindlabs.vercel.app/docs/detecting-classes-in-source-files#which-files-are-scanned)
- Preview

Author: rozsazoltan

src/docs/detecting-classes-in-source-files.mdx

@@ -135,6 +135,7 @@ As long as you always use complete class names in your code, Tailwind will gener
 Tailwind will scan every file in your project for class names, except in the following cases:
 
 - Files that are in your `.gitignore` file
+- Files in the `node_modules` directory
 - Binary files like images, videos, or zip files
 - CSS files
 - Common package manager lock files