Add maps for all colors, document how to extend color utilities (#32319)

* Add maps for all colors, document how to extend color utilities

* Updates to make this more functional for v5

- Moves color maps into _variables.scss for now (felt weird to split them out when the variables are there)
- Adds a new function, map-merge-multiple(), and updates docs that reference combo-map
- Updates code snippet and docs guidance about how to extend the color utils to handle this

* Update site/content/docs/5.0/customize/color.md

Co-authored-by: Gaël Poupard <ffoodd@users.noreply.github.com>
Co-authored-by: XhmikosR <xhmikosr@gmail.com>

scss/_functions.scss

@@ -55,6 +55,16 @@
   @return $result;
 }
 
+// Merge multiple maps
+@function map-merge-multiple($maps...) {
+  $merged-maps: ();
+
+  @each $map in $maps {
+    $merged-maps: map-merge($merged-maps, $map);
+  }
+  @return $merged-maps;
+}
+
 // Replace `$search` with `$replace` in `$string`
 // Used on our SVG icon backgrounds for custom forms.
 //