Clicking "API" in the System Portal brings you to the Actors Documentation page:
Here you find five actor categories, only the first two are supported:
CloudAPI actors: the API publicly available to all users
CloudBroker actors: API available only to administrators, providing the functionality as available from the Cloud Broker Portal and the Grid Portal
The three other API groups are only for internal use:
JumpScale actors: internal API for managing the firewalls
LibCloud actors: internal API used during configuration and setup
System actors: all other internal system APIs
All the APIs are described in the OpenAPI Specification.
Through the Swagger UI you can interact with them, here for the Add User API endpoint:
While all documentation for using this API is nicely included in this Swagger API console, you can also create a static HTML web page for the API documentation as documented in Create Static HTML Page for API Documentation.
In order to get stared with the Cloud API check Getting started with the OpenvCloud Cloud API.