docs: add arch decision record for the migration path for legacy user grouping mechanisms

docs/decisions/0005-migration-path-for-legacy-mechanisms.rst

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+5. Migration Path for Legacy User Grouping Mechanisms
+#####################################################
+
+Status
+******
+
+**Draft** - 2025-06-03
+
+Context
+*******
+
+Open edX currently uses several user grouping mechanisms (Cohorts, Teams,
+Enrollment Track Groups), each with its own logic, storage, and integration
+points. This fragmentation results in:
+
+- Complicates maintenance and evolution.
+- Makes it difficult to implement new functionality.
+- Limits interoperability and extensibility.
+
+Two migration paths were evaluated to transition to a unified grouping system:
+
+- **Cross-System Synchronization**: creates an abstraction layer that
+  translates the new model to the legacy mechanisms.
+- **Behavior Replication**: builds a new and independent system that replicates
+  the observable behavior of the legacy mechanisms without integrating with
+  them.
+
+Decision
+********
+
+We select the behavior replication approach, eliminating direct dependencies on
+legacy systems. This choice enables a simpler, cleaner architecture with:
+
+- Full independence from legacy mechanisms from day one.
+- Elimination of complex synchronization or integration layers.
+- Reduced technical debt and maintenance costs during migration.
+
+Existing user-facing functionalities will be replicated in the new model, with
+migration executed in clear, isolated phases to minimize risk. Activation will
+be controlled via feature flags, configurable per course, organization, or
+platform.
+
+See `ADR 6 <docs/decisions/0006-replication-of-legacy-mechanisms-behavior.rst>`_
+for detailed rationale.
+
+Consequences
+************
+
+- The new system can evolve independently, allowing greater flexibility.
+- The responsibility for replicating legacy behavior lies entirely within the
+  new model, which must be thoroughly validated.
+- The transition can be carried out gradually, implementing one functionality
+  at a time, allowing individual behavior validation and more targeted testing.
+- Both new and legacy systems can coexist during rollout, avoiding user
+  disruption.
+- Legacy systems will be fully deprecated and removed post-transition,
+  improving maintainability and extensibility.
+
+Rejected Alternatives
+*********************
+
+Cross-System Synchronization
+============================
+
+This proposal involved creating a new unified model while maintaining indirect
+synchronization with the legacy mechanisms through an abstraction layer. This
+layer would be responsible for:
+
+- Translating the logic of the new system to Cohorts, Teams, and Enrollment
+  Track Groups.
+- Ensuring backward compatibility during the entire transition.
+- Enabling a gradual adoption while maintaining functional consistency with the
+  legacy systems.
+
+Reasons it was rejected:
+
+- Significant increase in technical complexity: maintaining bi-directional
+  synchronization between two systems introduces risk of errors, logic
+  duplication, and hard-to-debug issues.
+- Higher maintenance cost: any change in the platform or legacy models would
+  also require updating the synchronization layer.
+- Interference with the evolution of the new model: depending on legacy systems
+  limits the ability of the new system to introduce more flexible criteria or
+  rules.
+- Greater difficulty in isolating and testing the new system: requiring the
+  presence of legacy systems makes independent validation of the new model more
+  complex.
+- Legacy cleanup becomes harder: as long as active synchronization exists,
+  legacy code cannot be removed without breaking dependencies.
+
+References
+**********
+
+- `Cross-System Synchronization Proposal <https://openedx.atlassian.net/wiki/x/AoBhJwE>`_
+- `Behavior Replication Proposal <https://openedx.atlassian.net/wiki/x/AgDiKgE>`_