* PEP 675: gentler intro for the Motivation section.
* PEP 675: explain usage stats.
* PEP 675: remove duplicate Backwards Compatibility heading.
There was already a Backwards Compatibility heading for Type Inference.
So, I just inlined the second one into the Runtime Behavior section.
* PEP 675: link to Other Uses in the Motivation.
* PEP 675: clarify that we insert a new type in the hierarchy.
* PEP 675: Make Backwards Compatibility a top-level section.
We had two "sections" - one for the type inference and one for the
runtime behavior. Move them both into one top-level section.
* PEP 675: Change case of Runtime Behavior.
* PEP 675: Fix and widen hyperlink text.
CAM Gerlach
Pradeep Kumar
Adam Turner