EPIC: Document Design Decision process

### What

Currently the Change Management process has a template https://eclipse-score.github.io/process_description/main/process_areas/change_management/guidance/change_management_decision_record_template.html - but it is not described in the process how to use it. Especially:
- Who needs to approve, what is the review process?
- How to plan the consequences of a decision?
- Are there different variants for different affected areas?
- How do we verify that a design decision is respected?

See also https://github.com/orgs/eclipse-score/discussions/2234#discussioncomment-15869210

### How

_No response_

### Estimates for realization

1 week

### Category

- [ ] Affects Detailed Design

### Requirements / Architecture

- [x] Requirements / Architecture are not affected by this change?