Update breadcrumb docs and dividers

- Add CSS custom property with fallback to Sass variable
- Update docs to mention the new CSS custom property
- Rewrite some of the docs to use divider instead of separator, and add some context here and there

scss/_breadcrumb.scss

@@ -18,7 +18,7 @@
       float: left; // Suppress inline spacings and underlining of the separator
       padding-right: $breadcrumb-item-padding-x;
       color: $breadcrumb-divider-color;
-      content: escape-svg($breadcrumb-divider);
+      content: var(--bs-breadcrumb-divider, escape-svg($breadcrumb-divider));
     }
   }