docs: add ADR for using the edx-platform CourseApp Django app (#49)

This commit adds an ADR to document our decision to use the edx-platform CourseApp Django app to register a Learning Assistant CourseApp plugin with the platform. The impact of this is that this feature will be exposed via the CourseApp REST API, and a card for this feature will appear on the Pages & Resources page.

docs/decisions/0003-courseapp-use.rst

@@ -0,0 +1,58 @@
+3. CourseApp Use
+################
+
+Status
+******
+
+**Accepted** *2024-01-08*
+
+Context
+*******
+
+We are adding the ability for course teams to manually enable and disable the Learning Assistant in their courses via
+a card in the `Pages & Resources page`_ of Studio, provided that the Learning Assistant is enabled and available in
+a given course.
+
+The `Pages & Resources page`_ is powered on the backend by the edX Platform `CourseApp Django app`_. With one
+exception in the case of Xpert Unit Summaries, each card on the `Pages & Resources page`_ has a corresponding instance
+of the `CourseApp plugin`_ configured in the `edx-platform repository`_. The `Pages & Resources page`_ can then make
+calls to the `backend CourseApp REST API`_ to get necessary information about the feature described by the plugin.
+
+The one exception to this behavior is the ``Xpert unit summaries card``, which uses a `Javascript object`_ stored in a
+static file in the `frontend-app-course-authoring repository`_ to describe the options that would otherwise be returned
+by the `backend CourseApp REST API`_ using a corresponding ``CourseApp`` plugin.
+
+Currently, the only ``CourseApp`` plugins registered in the platfrom are those that are defined within the
+`edx-platform repository`_. There are currently no plugins that are defined outside of the `edx-platform repository`_.
+
+Decision
+********
+
+* We will define an instance of the `CourseApp plugin`_ in this repository to describe the Learning Assistant feature.
+* We will register the `CourseApp plugin`_ as an entrypoint with the ``openedx.course_app`` key so that the `CourseApp
+  Django app`_ will be able to pick up the plugin if the Learning Assistant plugin is installed into ``edx-platform``.
+
+Consequences
+************
+
+* The Learning Assistant ``CourseApp`` plugin will only be registered with the `CourseApp Django app`_ if, and,
+  therefore, the Learning Assistant card on the `Pages & Resources page`_ will only be visible if, the Learning
+  Assistant plugin is installed into ``edx-platform``.
+* It will become slightly more difficult to know which ``CourseApps`` are available simply by reading the code, because
+  this ``CourseApp`` plugin is not stored in the `edx-platform repository`_.
+
+Rejected Alternatives
+*********************
+
+* We decided not to add in a custom backend REST API for exposing the ability to introspect the Learning Assistant
+  feature and enable and disable it. There already exists the `CourseApp Django app`_ for this purpose, and using it
+  allows us to avoid writing a lot of ad hoc code.
+
+.. _backend CourseApp REST API: https://github.com/openedx/edx-platform/blob/master/openedx/core/djangoapps/course_apps/rest_api/v1/views.py#L80
+.. _CourseApp Django app: https://github.com/openedx/edx-platform/tree/master/openedx/core/djangoapps/course_apps
+.. _CourseApp plugin: https://github.com/openedx/edx-platform/blob/master/openedx/core/djangoapps/course_apps/plugins.py#L15
+.. _edx-platform: https://github.com/openedx/edx-platform
+.. _edx-platform repository: https://github.com/openedx/edx-platform
+.. _frontend-app-course-authoring repository: https://github.com/openedx/frontend-app-course-authoring/tree/master
+.. _Javascript object: https://github.com/openedx/frontend-app-course-authoring/blob/master/src/pages-and-resources/xpert-unit-summary/appInfo.js
+.. _Pages & Resources page: https://github.com/openedx/frontend-app-course-authoring/tree/master/src/pages-and-resources