layout/z-index.md: retrieve z-index variables on build time

site/content/docs/4.3/layout/z-index.md

@@ -12,13 +12,19 @@ These higher values start at an arbitrary number, high and specific enough to id
 We don't encourage customization of these individual values; should you change one, you likely need to change them all.
 
 {{< highlight scss >}}
-$zindex-dropdown:          1000 !default;
-$zindex-sticky:            1020 !default;
-$zindex-fixed:             1030 !default;
-$zindex-modal-backdrop:    1040 !default;
-$zindex-modal:             1050 !default;
-$zindex-popover:           1060 !default;
-$zindex-tooltip:           1070 !default;
+{{< zindex.inline >}}
+{{- $file := readFile "scss/_variables.scss" -}}
+{{- $matches := findRE `\$zindex\-.+;` $file -}}
+
+{{- if (eq (len $matches) 0) -}}
+{{- errorf "Got no matches for $zindex- in %q!" $.Page.Path -}}
+{{- end -}}
+
+{{- range $matches }}
+{{ . | replaceRE "\\s{13}" "    " }}
+{{- end -}}
+
+{{< /zindex.inline >}}
 {{< /highlight >}}
 
 To handle overlapping borders within components (e.g., buttons and inputs in input groups), we use low single digit `z-index` values of `1`, `2`, and `3` for default, hover, and active states. On hover/focus/active, we bring a particular element to the forefront with a higher `z-index` value to show their border over the sibling elements.