feat(theme/book): Scroll to top when clicking the page title (#613)

Common pattern, especially on mobile devices where the page can be quite long.

src/theme/book.css

@@ -220,6 +220,7 @@ html:not(.sidebar-visible) #menu-bar:not(:hover).folded > #menu-bar-sticky-conta
   overflow: hidden;
   -o-text-overflow: ellipsis;
   text-overflow: ellipsis;
+  cursor: pointer;
 }
 .nav-chapters {
   font-size: 2.5em;

src/theme/book.js

@@ -522,6 +522,14 @@ function playpen_text(playpen) {
     });
 })();
 
+(function scrollToTop () {
+    var menuTitle = document.querySelector('.menu-title');
+
+    menuTitle.addEventListener('click', function () {
+        document.scrollingElement.scrollTo({ top: 0, behavior: 'smooth' });
+    });
+})();
+
 (function autoHideMenu() {
     var menu = document.getElementById('menu-bar');
 

src/theme/stylus/menu.styl

@@ -38,4 +38,5 @@ html:not(.sidebar-visible) #menu-bar:not(:hover).folded > #menu-bar-sticky-conta
     white-space: nowrap
     overflow: hidden
     text-overflow: ellipsis
+    cursor: pointer;
 }