docs: add ADR for UI mixin refactor

docs/decisions/0003-ui-mixins.rst

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+#######################
+0003 UI Mixins
+#######################
+
+Status
+******
+
+Accepted (September 2023)
+
+Context
+*******
+
+Previously, ORA frontend development has been challenging. ORA frontends were built
+using a combination of a placeholder Django template and injection of per-step HTML.
+Additionally, the way we built the context and HTML per section mixed business and
+presentation logic in complicated ways. This made refactoring and accessing that
+underlying data in any other context challenging.
+
+Along the way to creating a new ORA UI, we decided on some refactoring to aid access
+to that data and allow us to reuse that code across UI implementations.
+
+Decisions
+*********
+
+- We will standalone APIs for accessing data on an ORA block, placed in
+  ``openassessment/xblock/apis``. These APIs will be instantiated at block load and act
+  as the primary way we access data / functions on the ORA block.
+- We will move existing presentation code into ``openassessment/xblock/ui_mixins/legacy``,
+  refactoring to leverage our newly-created APIs for data access. 
+- New UI variants will also be created in ``openassessment/xblock/ui_mixins`` as separate
+  folders.
+
+Consequences
+************
+
+Standalone data / utility APIs
+==============================
+
+Splitting the data / utilities out of existing mixins and into a standalone location
+allows us to access that data across multiple contexts, without having to duplicate
+business logic.
+
+These APIs, in addition to serving our legacy UI views can also be leveraged in future
+UI views or other contexts that need access to that data or functions.
+
+Having those APIs and business logic in a single place will help us keep behavior from
+unintentionally drifting between UIs or other endpoints.
+
+Refactoring legacy presentation code
+====================================
+
+Refactoring our legacy presentation will allow us to leverage and exercise our APIs
+ensuring behavior is not altered from before refactor to after refactor.
+
+Moving this legacy presentation code will also allow us to decouple it from its
+currently tightly-coupled location inside of the openassessment block.
+
+Location for new UI variants
+============================
+
+Creating a specific location for UI variants (past and present) helps clearly
+separate our presentation and business concerns.
+
+Additionally, this gives us a well-defined pattern for introducing new UIs or iterating
+on our UIs moving forward.