CI: add docs

Feature Requests

Description

The CI process should be better documented so that others can understand what is happening.

User Stories

As maintainer of the project I need to be clear about what's happening in the CI in order to understand and fix problems.

Classification

Is this feature an enhancement of existing code or a completely new feature?

  • enhancement
  • new feature

/cc Mathias Göbel, Frank Schneider, Michelle Weidling