Add high-level documentation of system components and functions

[KlaasH]

This project has a fair number of components, and the overall shape of what they are and how they fit together is not currently documented in the main README or the ADRs, and having that written down is going to be important for enabling new people to work on the project.

So the task is to create a new documentation page cataloguing the components of the system and describing, in low-to-moderate detail, how they're set up and how they relate to each other.