Add definitions section: Maintainability

What does this MR do and why?

For #426255 (closed)

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

Note:

We have already used "maintainability" in our goals, so we absolutely MUST define that word so we can measure our success. This may invite lots of bikeshedding, but we can't throw around "maintainability" without some measurable definition.

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.