PEP 605: Change ABI management to encourage pre-built wheels (#1190)

* Change ABI management to encourage pre-built wheels
* Actually dedicate a subsection to the 2 year cadence
* Update naming discussion for proposal changes
* Add discussion section for the pre-freeze flag
* Make the abstract much shorter (as suggested by Antoine Pitrou)
* Move example time line up and add pictures (based on PEP 602 images)
* Clarify 4 month window for X.Y.0a1 preparation

Co-Authored-By: Steve Dower <[email protected]>

Author: ncoghlan