Add Django 4.0 official documentation (#8464)

books/free-programming-books-langs.md

@@ -2014,6 +2014,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Django Official Documentation](https://buildmedia.readthedocs.org/media/pdf/django/2.2.x/django.pdf) (PDF) (2.2)
 * [Django Official Documentation](https://buildmedia.readthedocs.org/media/pdf/django/3.1.x/django.pdf) (PDF) (3.1)
 * [Django Official Documentation](https://buildmedia.readthedocs.org/media/pdf/django/3.2.x/django.pdf) (PDF) (3.2)
+* [Django Official Documentation](https://buildmedia.readthedocs.org/media/pdf/django/4.0.x/django.pdf) (PDF) (4.0)
 * [Django Rest Framework](https://riptutorial.com/Download/django-rest-framework.pdf) - Compiled from StackOverflow Documentation (PDF)
 * [Django RESTful Web Services](https://www.packtpub.com/free-ebooks/django-restful-web-services) - Gaston C. Hillar (Packt account *required*)
 * [Django Web Framework (Python)](https://developer.mozilla.org/en-US/docs/Learn/Server-side/Django) - MDN contributors