[HW-1] added docs

docs/homework-1/img/cart-cart-item-delete.plantuml

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+@startuml
+
+actor User as u
+collections Cart as c
+database CartStorage as cs
+
+u -> c : DELETE /user/<user_id>/cart/<sku_id>
+activate c
+c -> cs : cart.DeleteItem(userID, skuID)
+c -> u : Response: 204 No Content
+deactivate c
+
+@enduml