gh-95913: Prepare remaining Whatsnew sections for editing (GH-98342)

* Add line breaks & ref targets to Whatsnew to prepare for future changes

* Use standard heading underbar symbols for H4 sections

* Flatten Porting subsection; clarify scope of/link Python->CAPI sections

* Move C API pending deprecations to C API section, to match the others
(cherry picked from commit 0a0c7e5a7a516607a1268c310a2e05d574a71efc)

Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>