Revert "Replace anchor.js with jekyll-anchor-headings."

This reverts commit b04f97f60b.

site/docs/4.1/assets/js/src/application.js

@@ -10,7 +10,7 @@
  * details, see https://creativecommons.org/licenses/by/3.0/.
  */
 
-/* global ClipboardJS: false, bsCustomFileInput: false */
+/* global ClipboardJS: false, anchors: false, bsCustomFileInput: false */
 
 (function ($) {
   'use strict'
@@ -101,6 +101,10 @@
         .tooltip('_fixTitle')
     })
 
+    anchors.options = {
+      icon: '#'
+    }
+    anchors.add('.bd-content > h2, .bd-content > h3, .bd-content > h4, .bd-content > h5')
     $('.bd-content').children('h2, h3, h4, h5').wrapInner('<span class="bd-content-title"></span>')
 
     bsCustomFileInput.init()

site/docs/4.1/assets/scss/_anchor.scss

@@ -1,15 +1,10 @@
-.bd-anchor {
-  padding-left: .25rem;
+.anchorjs-link {
   font-weight: 400;
-  color: $link-color;
-  opacity: 0;
-  @include transition(opacity .16s linear);
+  color: rgba($link-color, .5);
+  @include transition(color .16s linear);
 
-  :hover > &,
-  &:hover,
-  &:focus {
+  &:hover {
     color: $link-color;
     text-decoration: none;
-    opacity: 1;
   }
 }