[API clean up] Clean up api/shipments endpoints

[luisramos0]

What we should change and why (this is tech debt)

Api/shipments comes from Spree and is used in the admin orders edit page to add variants to the order.
In OFN one order has only one shipment so shipments are actually dummy in OFN.

We can keep the shipments or we can hide the shipments API. Long term, it may be a bad idea to keep shipments public as we may want to get rid or change them as we abandon Spree...
An alternative would be to have a nice API under /order/{order_number}/contents

So, maybe the decision is to keep shipments api for the admin orders edit page as is and wait for an opportunity to build a new API, probably under /order/{order_number}/contents. This will come from a API feature request to use the API to create orders from an external client.

Context

This came up from the API documentation work #3001

Impact and timeline

I conclude that we should NOT have external clients using /api/shipments and if anyone wants to add items to an order, we need to build new endpoints. So I think this issue to clean up shipments is low value and will probably never be done.

I keep it open for reference.... as this topic will arise very soon.