Docs: add documentation on frontend architecture

[olavis]

Describe Problem

The chapter Architecture overview claims to give you.. well an overview, but while it briefly explains concepts about the backend, it doesn't even mention anything about the client.

Suggest Changes

Make clear boundaries between frontend and backend, while the "overview" section should document the dependencies between them and how they communicate.

Provide Examples

The architecture overview section has illustrations that gives the impression that the frontend (aka one half of the application) is just some "painting on top" that has no architecture or structure: