Breadcrumb docs: Add MDN CSS doc links

docs/components/breadcrumb.md

@@ -6,7 +6,7 @@ group: components
 
 Indicate the current page's location within a navigational hierarchy.
 
-Separators are automatically added in CSS through `:before` and `content`.
+Separators are automatically added in CSS through [`::before`](https://developer.mozilla.org/en-US/docs/Web/CSS/::before) and [`content`](https://developer.mozilla.org/en-US/docs/Web/CSS/content).
 
 {% example html %}
 <ol class="breadcrumb">