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 gp-libs 0.0.18
  linking to the project PR

Author: tony

CHANGES

@@ -41,6 +41,7 @@ $ uvx --from 'gp-libs' --prerelease allow gp-libs
 ### Documentation
 
 - Migrate docs deployment to AWS OIDC authentication and AWS CLI
+- Visual improvements to API docs from [gp-sphinx](https://gp-sphinx.git-pull.com)-based Sphinx packages (#67)
 
 ## gp-libs 0.0.17 (2025-12-07)