add doc

doc/themes/next/source/js/scroll-cookie.js

@@ -0,0 +1,25 @@
+$(document).ready(function() {
+
+  /* global Cookies */
+
+  // Set relative link path (without domain)
+  var rpath = window.location.href.replace(window.location.origin, '');
+
+  // Write position in cookie
+  var timeout;
+  $(window).on('scroll', function() {
+    clearTimeout(timeout);
+    timeout = setTimeout(function() {
+      Cookies.set('scroll-cookie', $(window).scrollTop() + '|' + rpath, { expires: 365, path: '' });
+    }, 250);
+  });
+
+  // Read position from cookie
+  if (Cookies.get('scroll-cookie') !== undefined) {
+    var cvalues = Cookies.get('scroll-cookie').split('|');
+    if (cvalues[1] === rpath) {
+      $(window).scrollTop(cvalues[0]);
+    }
+  }
+
+});