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 libvcs 0.40.x
  linking to the project PR

Author: tony

CHANGES

@@ -20,6 +20,11 @@ $ uv add libvcs --prerelease allow
 _Notes on the upcoming release will go 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 (#522)
+
+
 ### What's new
 
 #### pytest plugin: Improve typings (#521)