Add a definition of done for technical breakdowns
Context
We don't have an agreed-upon method to determine whether a technical breakdown is done. Especially when the technical planning & related work are scheduled for the same milestone, this can make it unclear when we're ready to start writing & merging code.
Changes in this MR
- defines the goals of technical planning
- proposes a series of approvals as definition of done
- adds a few bonus comments to the template