Fix reStructuredText link syntax in introduction.txt

[Shrikantgiri25]

Fixes #492

Summary

This PR fixes the broken reStructuredText (RST) link syntax in docs/introduction.txt that prevented the Celery documentation link from rendering correctly.

Changes Made

1. Fixed reference syntax – Replaced :ref:Celery documentation <django-celery-results> with proper external link syntax.
2. Matched reference name – Ensured the link text and reference name both use Celery documentation.
3. Updated URL – Changed /master/ to /latest/ to point to the stable Celery documentation.

Before

The installation instructions for this extension is available from the :ref:Celery documentation <django-celery-results>.

After

.. _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`_.

Why This Fix is Needed

• The :ref: role is meant for internal Sphinx cross-references, not external URLs.
• The previous link pointed to /master/, which no longer exists and should reference /latest/.
• This fix ensures the installation link renders correctly and leads to the stable, existing documentation.

Testing

• Verified that the RST syntax follows standard external link formatting.
• Confirmed the reference name matches the visible link text.
• Checked that the updated URL correctly opens the stable /latest/ documentation.
• Built the documentation locally using the provided .bat file to confirm the fix renders correctly.
• Opened the generated index.html and clicked on the “Celery documentation” link — it now redirects properly to:
  https://docs.celeryq.dev/en/latest/django/first-steps-with-django.html#django-celery-results

Steps to Reproduce and Verify Locally

1. Navigate to the repo root:

   cd django-celery-results

2. Build the documentation using the Windows batch file:

   docs\make.bat html

3. Open the generated documentation:

   start docs\_build\html\index.html

4. In your browser, click the “Celery documentation” link under the Installing section.
   It now correctly opens the stable /latest/ Celery documentation page.

Impact

• Fixes the broken link in the documentation.
• No code or functionality changes.
• Improves user experience when accessing installation instructions.