Replace anchor.js with jekyll-anchor-headings.

This allows us to generate the anchor links on build time.

https://github.com/allejo/jekyll-anchor-headings

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, anchors: false, Holder: false, bsCustomFileInput: false */
+/* global ClipboardJS: false, Holder: false, bsCustomFileInput: false */
 
 (function ($) {
   'use strict'
@@ -101,10 +101,6 @@
         .tooltip('_fixTitle')
     })
 
-    anchors.options = {
-      icon: '#'
-    }
-    anchors.add('.bd-content > h2, .bd-content > h3, .bd-content > h4, .bd-content > h5')
     $('.bd-content > h2, .bd-content > h3, .bd-content > h4, .bd-content > h5').wrapInner('<div></div>')
 
     // Holder

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

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