Update Anchor.js and port over the existing styles.

Anchor.js 1.0.0 drops the need to use anchor.css, so we remove it
from the codebase and move any leftover styles over to docs.css.

Closes #16488 by merging it.

docs/assets/css/src/docs.css

@@ -1595,6 +1595,31 @@ h1[id] {
   }
 }
 
+/*
+ * AnchorJS Styles
+ */
+.anchorjs-link {
+  color: inherit;
+}
+
+@media (max-width: 480px) {
+  .anchorjs-link {
+    display: none;
+  }
+}
+
+*:hover > .anchorjs-link {
+  opacity: .75;
+  -webkit-transition: color .16s linear;
+       -o-transition: color .16s linear;
+          transition: color .16s linear;
+}
+
+*:hover > .anchorjs-link:hover,
+.anchorjs-link:focus {
+  text-decoration: none;
+  opacity: 1;
+}
 
 /*
  * Miscellaneous