Add basically API documentation

docs/concept/primary-device.md

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+# Primary device concept
+
+One (child) user can be assigned to multiple devices. Without synchronization,
+this can result in using the time limits once per device and not only once.
+Due to that, there is (by default) the limitation that a child must select
+one device for using limited Apps.
+
+There can be one or zero primary devices. This is required for the primary device
+switching process and for the initial state.
+
+## assign process
+
+- sync
+- try to assign the user using the [child API](../api/child.md)
+- if the user is assigned to a different device, request a logout using the [child API](../api/child.md) and retry it
+  - this should not be done in a loop without delays
+  - using the [websocket](./websocket.md) it is possible to get changes without polling
+  - requesting a sign out should be repeated all few seconds
+
+## unassign process
+
+- sync
+- unassign the user using the [child API](../api/child.md)