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/content/docs/5.2/utilities/z-index.md

@@ -0,0 +1,46 @@
+---
+layout: docs
+title: Z-index
+description: Use our low-level `z-index` utilities to quickly change the stack level of an element or component.
+group: utilities
+toc: true
+---
+
+## Example
+
+{{< added-in "5.3.0" >}}
+
+Use `z-index` utilities to stack elements on top of one another. Requires a `position` value other than `static`, which can be set with custom styles or using our [position utilities]({{< docsref "/utilities/position/" >}}).
+
+{{< callout >}}
+We call these "low-level" `z-index` utilities because of their default values of `-1` through `3`, which we use for the layout of overlapping components. High-level `z-index` values are used for overlay components like modals and tooltips.
+{{< /callout >}}
+
+{{< example class="bd-example-zindex-levels position-relative" >}}
+<div class="z-n1 position-absolute p-5 rounded-3"></div>
+<div class="z-0 position-absolute p-5 rounded-3"></div>
+<div class="z-1 position-absolute p-5 rounded-3"></div>
+<div class="z-2 position-absolute p-5 rounded-3"></div>
+<div class="z-3 position-absolute p-5 rounded-3"></div>
+{{< /example >}}
+
+## Overlays
+
+Bootstrap overlay components—dropdown, modal, offcanvas, popover, toast, and tooltip—all have their own `z-index` values to ensure a usable experience with competing "layers" of an interface.
+
+Read about them in the [`z-index` layout page]({{< docsref "/layout/z-index" >}}).
+
+## Component approach
+
+On some components, we use our low-level `z-index` values to manage repeating elements that overlap one another (like buttons in a button group or items in a list group).
+
+Learn about our [`z-index` approach]({{< docsref "/extend/approach#z-index-scales" >}}).
+
+## CSS
+
+### Sass map
+
+Customize this Sass map to change the available values and generated utilities.
+
+{{< scss-docs name="zindex-levels-map" file="scss/_variables.scss" >}}
+