Fix icon-link underline color referencing a nonexistent v5 variable

.icon-link relied on the removed --link-color-rgb and --link-opacity
variables, so its underline never picked up custom link colors. Use
color-mix() against --link-color instead.

Author: mdo

scss/helpers/_icon-link.scss

@@ -25,7 +25,7 @@ $icon-link-tokens: defaults(
     display: inline-flex;
     gap: var(--icon-link-gap);
     align-items: center;
-    text-decoration-color: rgba(var(--link-color-rgb), var(--link-opacity, .5));
+    text-decoration-color: color-mix(in oklch, var(--link-color) 50%, transparent);
     text-underline-offset: var(--icon-link-underline-offset);
     backface-visibility: hidden;
 

site/src/content/docs/helpers/icon-link.mdx

@@ -59,9 +59,9 @@ Customize the hover `transform` by overriding the `--bs-icon-link-transform` CSS
     Icon link
   </a>`} />
 
-Customize the color by overriding the `--bs-link-*` CSS variable:
+The icon link’s underline color is derived from `--bs-link-color`, so overriding it updates both the text and the underline together:
 
-<Example code={`<a class="icon-link icon-link-hover" style="--bs-link-hover-color-rgb: 25, 135, 84;" href="#">
+<Example code={`<a class="icon-link icon-link-hover" style="--bs-link-color: #198754;" href="#">
     Icon link
     <svg xmlns="http://www.w3.org/2000/svg" class="bi" viewBox="0 0 16 16" aria-hidden="true">
       <path d="M1 8a.5.5 0 0 1 .5-.5h11.793l-3.147-3.146a.5.5 0 0 1 .708-.708l4 4a.5.5 0 0 1 0 .708l-4 4a.5.5 0 0 1-.708-.708L13.293 8.5H1.5A.5.5 0 0 1 1 8z"/>