Add basically API documentation

docs/api/child.md

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+# /child
+
+This endpoint is used for by devices which are used by a child.
+
+## POST /child/add-device
+
+Use this during the setup to add a device to an existing family using an add device code.
+
+### request
+
+see [this JSON schema](../schema/registerchilddevicerequest.md)
+
+### response
+
+On a invalid request body: HTTP status code 400 Bad request
+
+On a invalid add device token: HTTP status code 401 Unauthorized
+
+On success: a JSON object with the properties ``deviceAuthToken`` and ``ownDeviceId``,
+both of the type string
+
+## POST /child/update-primary-device
+
+Use this to (un)set the calling device as primary device.
+
+### request
+
+see [this JSON schema](../schema/updateprimarydevicerequest.md)
+
+### response
+
+On a invalid request body: HTTP status code 400 Bad request
+
+On a invalid auth token: HTTP status code 401 Unauthorized
+
+If the transmitted ``currentUserId`` is empty or does not match the current user of the device:
+HTTP status code 409 Conflict
+
+If the transmitted ``currentUserId`` does not match a valid user: HTTP status code 409 Conflict
+
+If trying to unset the device as primary device and the device is not the current device of the user:
+HTTP status code 409 Conflict
+
+Otherwise: a JSON object with the property ``status`` (string)
+
+``status`` can have the following values:
+
+- ``assigned to other device`` (if the user is assigned to an other device)
+- ``requires full version``
+- ``success``
+
+### see
+
+- [primary device concept](../concept/primary-device.md)
+
+## POST /child/logout-at-primary-device
+
+Use this to request unsetting the current primary device of the user
+which is currently assigned to the calling device. This triggers a websocket
+message to the current primary device of the user.
+
+### request
+
+see [this JSON schema](../schema/requestwithauthtoken.md)
+
+### response
+
+On a invalid request body: HTTP status code 400 Bad request
+
+On a invalid auth token: HTTP status code 401 Unauthorized
+
+If there is no user assigned to the calling device or
+if there is no existing current primary device: HTTP status code 409 Conflict
+
+On success: ``{"ok": true}``
+
+### see
+
+- [primary device concept](../concept/primary-device.md)