docs: add ADR for pulling cc code out of platform (#260)

* docs: add ADR for pulling cc code out of platform

docs/decisions/0008-pull-commerce-coordinator-code-out-of-platform.rst

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+8. Pull Commerce Coordinator related code out of edx-platform
+#############################################################
+
+Status
+******
+
+**Draft**
+
+Context
+*******
+
+Axim has marked Commerce Coordinator related code in edx-platform as ``business-related``. For Reference see `PR`_
+
+.. _PR: https://github.com/openedx/edx-platform/pull/35203
+
+As a result of this, we need to pull out all the references of ``commerce-coordinator`` from ``edx-platform`` while also finding a way to override the ``edx-platform`` code from our tooling to ensure that ``commerce-coordinator`` functionality remains intact.
+
+Decision
+********
+
+We have decided to use `Django App Plugins`_ from `edx_django_utils`_ to override the `EcommerceService().payment_page_url`_ and `lms.djangoapps.commerce.util.refund_seat`_ functions/methods in ``edx-platform`` in which currently we have implemented ``commerce-coordinator`` specific code.
+
+`Django App Plugins`_ provide us with a way to override the application code without modifying the original codebase. Specifically we will be using the `pluggable_overrides`_ utility which is designed to override any method/function to point to an alternative implementation.
+As a result of this we will be able to pull out the ``commerce-coordinator`` related code from ``edx-platform`` into a private pluggable application and still be able to use the functionality provided by ``commerce-coordinator``.
+
+For the implementation, we will be creating a new pluggable application that will house all the business specific ``commerce-coordinator`` related code in overridden functions. This new pluggable application will be installed with `EDXAPP_PRIVATE_REQUIREMENTS`_ from `edx-internal`_.
+This solution also ensures that regular flow for the community remains unaffected.
+
+.. _Django App Plugins: https://github.com/openedx/edx-django-utils/tree/master/edx_django_utils/plugins#django-app-plugins
+.. _edx_django_utils: https://github.com/openedx/edx-django-utils
+.. _pluggable_overrides: https://github.com/openedx/edx-django-utils/blob/master/edx_django_utils/plugins/pluggable_override.py
+.. _EDXAPP_PRIVATE_REQUIREMENTS: https://github.com/edx/edx-internal/blob/master/ansible/vars/edx.yml#L38
+.. _edx-internal: https://github.com/edx/edx-internal/
+.. _EcommerceService().payment_page_url: https://github.com/openedx/edx-platform/blob/master/lms/djangoapps/commerce/utils.py#L103
+.. _lms.djangoapps.commerce.util.refund_seat: https://github.com/openedx/edx-platform/blob/master/lms/djangoapps/commerce/utils.py#L236
+
+Rejected Alternatives
+*********************
+
+A lengthy R&D was undertaken to look at several options for pulling out the commerce coordinator related code from ``edx-platform``.
+
+Alternatives that were investigated and rejected:
+
+- Using EduNext's Ecommerce extension
+    - Rejected due to:
+        - The extension solely deals with adding more payment gateways to the ecommerce ecosystem.
+        - The extension does not provide a way to override the existing code blocks in ``edx-platform``.
+        - The extension is not actively maintained.
+- Generic Flag names in edx-platform
+    - Rejected due to:
+        - This approach is not scalable.
+        - This approach will not provide a way to override the existing code blocks in ``edx-platform``.