[FC-0118] docs: add ADR for modulestore crud api with custom serializers

[taimoor-ahmed-1]

Description

Adds ADR-039 proposing the pattern for exposing modulestore CRUD operations (courses and blocks) via Django REST Framework.

Since modulestore is backed by MongoDB/split rather than Django ORM models, standard ModelSerializer patterns don't apply. This ADR documents the decision to use explicit, non-model DRF serializers paired with a service layer for authorization and business logic, and custom get_object/get_queryset implementations in ViewSets that delegate to modulestore() calls.

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[Abdul-Muqadim-Arbisoft]

Please correct the file name from "ADR-039-modulestore-crud-apis-with-custom-serializers.rst" to "ADR-0039-modulestore-crud-apis-with-custom-serializers.rst" to maintain consistent numbering format.