Document the architecture of k8up

[cimnine]

Summary

As an operator using k8up
I want to understand how it's bits and pieces work together
So that I can debug my problem with greater accuracy.

As a (new) developer of k8up
I want to understand the overall idea behind the code
So that I can wrap my around and find the best solution to integrate new features and fix bugs.

Context

In #474 (part of #359), the component wrestic, previously in it's own repository yet never actually independent, was integrated into the k8up codebase.

For a person fresh to the project, it would be great to understand where these two components live in the repository and how they interact with each other.

Out of Scope

• Changes to the code that are not documentation.
• Graphics for all the exception cases – focus on the main flow.
• Links to actual pieces of code, as they outdate pretty quickly.

Further links

• https://k8up.io/k8up/1.2.0/explanations/architecture.html might be a good start

Acceptance criteria

• The interaction of restic / wrestic / k8up is made clear as good as possible.

Implementation Ideas

• Some graphical overview would be nice, like a component diagram or a sequence diagram.
• A high level overview of the folders and/or go modules that are relevant for the operator module.
• A high level overview of the folders and/or go modules that are relevant for the restic module.