Add z-index utilities, .z-* (#37317)

* Add z-index utilities, .z-*

* Fix missing comma in _utilities.scss

* Fix bundlewatch

* Fix lint

* Update site/content/docs/5.2/utilities/z-index.md

Co-authored-by: Julien Déramond <julien.deramond@orange.com>

* custom appearance

* linter

Co-authored-by: Julien Déramond <juderamond@gmail.com>
Co-authored-by: Julien Déramond <julien.deramond@orange.com>

site/data/sidebar.yml

@@ -133,6 +133,7 @@
     - title: Text
     - title: Vertical align
     - title: Visibility
+    - title: Z-index
 
 - title: Extend
   icon: tools