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 libtmux (unreleased)
  linking to the project PR

Author: tony

CHANGES

@@ -40,6 +40,11 @@ $ uvx --from 'libtmux' --prerelease allow python
 _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 (#658)
+
+
 ### What's new
 
 #### pytest plugin: fixture reference documentation with autofixture directives (#656)