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