Add z-index for .toast-container (#35859)

* Add z-index for .toast-container

- Adds positioning and z-index to toast container so that it has a system-declared layer in relation to other components.
- Updates docs to use the class better

Fixes $34028

* Remove unnecessary z-index from docs examples

scss/_toasts.scss

@@ -20,6 +20,8 @@
 }
 
 .toast-container {
+  position: absolute;
+  z-index: $zindex-toast;
   width: max-content;
   max-width: 100%;
   pointer-events: none;

scss/_variables.scss

@@ -1041,6 +1041,7 @@ $zindex-modal-backdrop:             1050 !default;
 $zindex-modal:                      1055 !default;
 $zindex-popover:                    1070 !default;
 $zindex-tooltip:                    1080 !default;
+$zindex-toast:                      1090 !default;
 // scss-docs-end zindex-stack