Discussion/doc cart's behaviour

There are many nuances in the cart's mechanism. It would be good to document all those. Then we can think of optimizing current architecture of OCC API and Sparatacus code. We should also cover all those documented nuances with e2e tests.

For example:

• cart of anonymous user is merged into exiting user's cart after login
• for calls to OCC we use userId 'anonymous' for unsigned users and real userId for signed ones
• for calls to OCC we use cartId 'current' (or cart.code? I'm not sure) for signed users and cart.guid for unsigned users
• we shouldn't load cart from backend when it's not created (question: it may be OK for unsigned users, but for signed may lead to two active carts on different devices, see https://github.com/SAP/cloud-commerce-spartacus-storefront/issues/3592)
• we should re-load cart from backend after all actions, i.e. like add cart entry (question: can we optimize backend API to return cart as a result of adding cart entry?)
• when cart doesn't exist in backend, we should create it just before user adds the first item to the cart
• question: can/should a signed user have different carts on different devices? (I guess not)

It's an open discussion. So please share thoughts in comments.

No file or test path is named. Review the existing cart documentation, OCC API behavior, Spartacus cart code, and issue 3592, then resolve the open questions before documenting the agreed scenarios and adding e2e coverage.