Documentation: system dependencies

[llorllale]

We should document the system dependencies the adapters require to work and what they're used for. Something similar to this would work but focused solely on the adapters and their immediate dependencies.

Also detail what dependencies are required for each API... pointing someone to this doc for instance does not inform what dependencies are required on the /relyingparties endpoint.

[llorllale]

This task was motivated by someone trying out the adapter endpoints and encountering errors due to missing system dependencies.