docs: add CI docs

Feature

Summary

This MR provides a more extensive documentation of all things CI.

Does the result of the MR comply to our "definition of done"?

• Unit tests passed
• Code reviewed
• Acceptance criteria met
• Functional tests passed
• Non-Functional requirements met
• Product Owner accepts the User Story

Documentation

• I adjusted other parts of the documentation (if applicable)

Tests

• No, it is not possible.

Closes

Closes #176 (closed)

/cc Mathias Göbel, Frank Schneider, Michelle Weidling