policy: agree on `towncrier` changelog process for point (patch) releases

[bjlittle]

📰 Custom Issue

Reference: #7166

Reach a contributor agreement on how to proceed with whatsnew updates to the towncrier generated changelog for a point (patch) release.

Currently iris generates a single separate whatsnew for each major or minor release with point (patch) releases included within that major or minor whatsnew page.

The towncrier workflow (see #7116) automatically collates the changelog directory news fragment files and generates the major or minor release whatsnew page.

We now have the problem of injecting the point (patch) release highlights into the associated whatsnew page, along with any relevant changelog news fragments.

Initial options:

1. Follow the towncrier workflow and henceforth (breaking with prior tradition) generate a dedicated point (patch) whatsnew release page i.e., we use towncrier to build the point (patch) changelog from news fragment files.
2. Follow existing tradition and incorporate the point (patch) details within the already existing major or minor whatsnew page. However, this will require manually adding and formatting the fragments directly into the page.

There may be alternative workflows to explore.

In both proposed initial options, the dedicated hightlights.rst associated with the whatsnew page is still manually edited to include the point (patch) hightlights.