Add prev and next chapter navigation

js/header.js

@@ -0,0 +1,19 @@
+<script>
+// When the user scrolls the page, execute myFunction
+window.onscroll = function() {headerscroll()};
+
+// Get the header
+var header = document.getElementById("NavHeader");
+
+// Get the offset position of the navbar
+var sticky = header.offsetTop;
+
+// Add the sticky class to the header when you reach its scroll position. Remove "sticky" when you leave the scroll position
+function headerscroll() {
+  if (window.pageYOffset > sticky) {
+    header.classList.add("sticky");
+  } else {
+    header.classList.remove("sticky");
+  }
+}
+</script>