Populate sphinx documentation

[diego-plan9]

With #82 in place, we can now populate the sphinx documentation (so it can be included from qiskit-ibm-runtime).

Ideally, this consist of:

• decide which is the public interface, and ensure all items from the public interface are on it
• provide a single entry point (ie. single .rst document) that can be included in qiskit-ibm-runtime documentation, and from which all documented items can be reached (directly or indirectly)

Acceptance criteria

The sphinx output of this repo contains a single entry point that allows all public interface items to be reached.