Add definitions: Done

What does this MR do and why?

For #427177 (closed)

Extracts discussion on definitions (in particular: our definition of done) from the issue to the living documentation for more effective collaboration.

Please note: Important caveat

This MR is by no means the final word. Moving this into the blueprint is intended to encourage more collaboration and refinement. We should use a bias for action and our iteration value to improve and expand this document.