Fix reStructuredText link syntax in introduction.txt

- Replace incorrect :ref: syntax with proper external link reference
- Change reference to match link text 'Celery documentation'
- Update URL from /master/ to /latest/ for stable documentation
- Ensures link renders correctly in generated documentation

Fixes #492

Author: Shrikant Giri

docs/includes/introduction.txt

@@ -19,4 +19,6 @@ it will be stored in :attr:`TaskResult.periodic_task_name <django_celery_results
 Installing
 ==========
 
-The installation instructions for this extension is available from the :ref:`Celery documentation <django-celery-results>`.
+.. _Celery documentation: https://docs.celeryq.dev/en/latest/django/first-steps-with-django.html#django-celery-results
+
+The installation instructions for this extension is available from the `Celery documentation`_.