Move from parent and child issues to epics

Why is this change being made?

This MR implements the Handbook change from the RFC to switch from Parent and Child issues to Epics

• It explains how we want to structure Epics, Child Epics and Issues
• The gotchas and workarounds when working with epics (especially when working with multiple top level groups)
• Where discussion and single source of truth should be

The reason why we want to switch to epics:

• Issue health status in epic tree seems very useful to get a better overview
• Generally a better overview of a project since issue status is always accurate
• It's easy to bulk edit issues
• It's easy to create issues in a bulk (which comes handy for breaking down issues)
• Nice overview of the roadmap
• Dogfooding 🐶 In the longterm epics will improve and be better than our parent/child solution

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
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