Define Solid connector interface for poc-fragestunde

[Jenziner]

The "Fragestunde" PoC (poc-fragestunde) has documented a proposal
for the Solid connector interface from the application's
perspective, see INTERFACES.md in the poc-fragestunde repo,
section "Solid-Connector".

Two use cases are foreseen: Bernasconi (triggered from within the
Fragenmanagement app) and Meier (independent, own Solid server).

Please review, adjust as needed, and document the actual interface
here in the rwp-solid-connector repo.