docs: ADR for new way to enable/disable teams (#28891)

Enabling or disabling team currently requires adding a team set/topic/group or removing them all. This ADR proposes a new mechanism.

lms/djangoapps/teams/docs/decisions/0001-field-to-store-enabled-disabled-status.rst

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+Add a new field to store enable/disabled status of teams
+--------------------------------------------------------
+
+Status
+======
+
+Proposal
+
+
+Context
+=======
+
+While implementing a UI to enable/disable teams we ran into issues due to the
+way teams is considered enabled or disabled, which is with the presence of one
+or more teamsets. So enabling requires adding a teamset and disabling requires
+removing all teamsets.
+
+Since we'd like to have a simple toggle to enable/disable teams this complicates
+the implementation significantly.
+
+Decisions
+=========
+
+We can add a new field called ``teams_enabled`` (or simply ``enabled``) to the
+``TeamsConfigField`` that stores the teams configuration in the course.
+
+Currently the ``is_enabled`` method on
+``openedx.core.lib.teams_config.TeamsConfig`` simply checks that the team_sets
+list isn't empty, it should switch to checking for the the ``enabled`` field
+instead. It can fall back to the existing behaviour so existing courses work
+as-is. However, any update to the value will add this boolean field.
+
+Alternative
+===========
+
+Any time teams is disabled, we can move the the existing teams configuration in
+the JSON from the key ``team_sets`` to ``teams_sets_inactive``. Existing code will
+ignore this field and disable the teams tab and other teams features.
+
+However the new UI can be made aware of this backup field and use it to
+reinitialise the teams config based on old data if needed. So if a course is
+set up with teams properly with a few teamsets, and the toggle is disabled,
+the data will be moved from ``team_sets`` / ``topics`` to ``team_sets_inactive`` and
+if teams is enabled again, the data will be moved from ``team_sets_inactive`` to
+``team_sets``.
+
+Optionally we can even allow configuring team sets this way before they are
+enabled. i.e. in a disabled state all teamset operation can be redirected to
+``team_sets_inactive``