gh-74929: PEP 667 C API documentation (gh-119379)

* 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

Python/executor_cases.c.h

@@ -1541,7 +1541,7 @@
         case _MAKE_CELL: {
             oparg = CURRENT_OPARG();
             // "initial" is probably NULL but not if it's an arg (or set
-            // via PyFrame_LocalsToFast() before MAKE_CELL has run).
+            // via the f_locals proxy before MAKE_CELL has run).
             PyObject *initial = GETLOCAL(oparg);
             PyObject *cell = PyCell_New(initial);
             if (cell == NULL) {