[3.13] gh-74929: PEP 667 C API documentation (gh-119892)

* Add docs for new APIs
* Add soft-deprecation notices
* Add What's New porting entries
* Update comments referencing `PyFrame_LocalsToFast()` to mention the proxy instead
* Other related cleanups found when looking for refs to the deprecated APIs

(cherry picked from commit 3859e09e3d92d004978dd838f0511364e7edfb94)

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>