Add documentation for python-decouple

# Utilise the following resources.
1. https://pypi.org/project/python-decouple/#why
2. https://simpleisbetterthancomplex.com/2015/11/26/package-of-the-week-python-decouple.html
3. https://medium.com/@nithinkvijayan/https-medium-com-nithinkvijayan-separating-django-application-config-and-secrets-from-code-python-decouple-e0787d2bcc2a

The issue asks to add documentation for python-decouple in a Django project. Start by reading the linked resources to understand the package's purpose for managing configuration and secrets. Look at the existing project structure to see where configuration is currently handled, likely in settings.py. The task is to write documentation explaining how to integrate and use python-decouple in this specific Django app.

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