docs(CHANGES) Visual API docs improvements via gp-sphinx

why: Capture the api-styling migration to the gp-sphinx package
stack in the unreleased notes.
what:
- Add a Documentation bullet under vcspull v1.59.x (unreleased)
  linking to the project PR

Author: tony

CHANGES

@@ -37,6 +37,10 @@ $ uvx --from 'vcspull' --prerelease allow vcspull
 _Notes on upcoming releases will be added here_
 <!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
 
+### Documentation
+
+- Visual improvements to API docs from [gp-sphinx](https://gp-sphinx.git-pull.com)-based Sphinx packages (#542)
+
 ## vcspull v1.58.1 (2026-03-29)
 
 ### Bug fixes