gh-95914: Add What's New item describing PEP 670 changes

[CAM-Gerlach]

Part of #95914 (and in turn, #95913).

As discussed with @erlend-aasland , adds an item to the C API -> Porting section of the Python 3.11 What's New document summarizing the changes proposed in PEP 670 (PEP-670), Converting macros to functions in the Python C API, and implemented by @vstinner in issue #89653 (and many linked PRs) and their impact on users. This is the last chunk needed before I can link every Summary item with a more detailed What's New entry, and then #95914 will finally be complete.

• Issue: gh-95914