Part of #95913

To ensure PRs to edit the remaining sections can all be submitted in parallel and don't introduce any merge conflicts with each other, this preliminary PR (which can hopefully be merged quickly) ensures there are two line breaks between each section (per standard convention), and adds the same standard ref target labels to each section as the previous to enable stable cross-referencing.

To note, this PR does not do so for any of the Improved Modules subsections, which will be done in a separate PR once the current batch of PRs adding/editing those are merged, so the rest can safely proceed in parallel without conflicts.

Additionally, it makes a few other straightforward but cross-cutting changes:

• Elide the redundant "Python API changes" subsection heading in the Porting section, given all the C API changes are discussed under the C API section
• Move the list of C APIs pending deprecation from the top-level Pending Removal section to a subsection of the C API section, to match all the others (Deprecated/Removed/Porting), and mention/link it accordingly
• Explicitly clarify that the top-level Deprecated/Pending Removal/Removed/Porting-section changes are referring to the Python API, and link the C API equivalents
• Use the standard H4 heading underbar symbols as stated in the devguide

• Issue: Edit What's New for Python 3.11 #95913