Refactor PlantUML diagram

The existing PlantUML diagram does not accurately depict the flow of information.

A user does not directly access the IAMFrontend. A user accesses an arbitrary frontend (or possibly backend, technically), and if the accessed frontend requires authentication itself or the user attempts an action that requires authentication, the user will be redirected to IAMFrontend, which will redirect to the login page served by Keycloak. After authentication, a series of two callbacks will happen, and the user will return to the originally accessed frontend.

There is an existing PlantUML here &5 (closed) which is a better approximation (but still not fully accurate) of what we are implementing.