Add architecture diagrams

[nscuro]

In order for us (and others) to better understand what we're building here, we should have architecture diagrams.

Preferably there should be multiple "resolutions" from a high-level overview to individual services. The topology diagrams we can generate using Kafka Streams will be helpful for the latter.

[nscuro]

An overview diagram was added in #228.

Topology diagrams are still pending currently.