As a software architect, I need to clearly describe and document the interfaces to the core services, so that they can be shared with other developers and used while developing these services in parallel.

Let's follow the OpenAPI Spec 3 (https://swagger.io/specification/) when describing the services. 

Some examples for reference:

https://opensource.ncsa.illinois.edu/confluence/display/BD/Brown+Dog+API

https://bd-api.ncsa.illinois.edu/docs/