fix(button): add custom properties and remove --ion-color overrides (#15463)

- adds custom properties 
- removes the overrides of `--ion-color-*` variables
- documents the properties

references #14808 references #14853 references #14850

core/src/components/button/button.tsx

@@ -231,6 +231,7 @@ function getColorClassMap(buttonType: string, color: string | undefined, fill: s
     [`${className}-${mode}`]: true,
   };
   if (color !== undefined) {
+    map[`ion-color`] = true;
     map[`ion-color-${color}`] = true;
   }
   return map;