[SPARK-8353] [DOCS] Show anchor links when hovering over documentation headers

This patch uses [AnchorJS](https://bryanbraun.github.io/anchorjs/) to show deep anchor links when hovering over headers in the Spark documentation. For example:

![image](https://cloud.githubusercontent.com/assets/50748/8240800/1502f85c-15ba-11e5-819a-97b231370a39.png)

This makes it easier for users to link to specific sections of the documentation.

I also removed some dead Javascript which isn't used in our current docs (it was introduced for the old AMPCamp training, but isn't used anymore).

Author: Josh Rosen <[email protected]>

Closes apache#6808 from JoshRosen/SPARK-8353 and squashes the following commits:

e59d8a7 [Josh Rosen] Suppress underline on hover
f518b6a [Josh Rosen] Turn on for all headers, since we use H1s in a bunch of places
a9fec01 [Josh Rosen] Add anchor links when hovering over headers; remove some dead JS code

Author: JoshRosen

LICENSE

@@ -950,3 +950,4 @@ The following components are provided under the MIT License. See project link fo
      (MIT License) scopt (com.github.scopt:scopt_2.10:3.2.0 - https://github.com/scopt/scopt)
      (The MIT License) Mockito (org.mockito:mockito-all:1.8.5 - http://www.mockito.org)
      (MIT License) jquery (https://jquery.org/license/)
+     (MIT License) AnchorJS (https://github.com/bryanbraun/anchorjs)

docs/_layouts/global.html

@@ -138,6 +138,7 @@ <h1 class="title">{{ page.title }}</h1>
 
         <script src="js/vendor/jquery-1.8.0.min.js"></script>
         <script src="js/vendor/bootstrap.min.js"></script>
+        <script src="js/vendor/anchor.min.js"></script>
         <script src="js/main.js"></script>
 
         <!-- MathJax Section -->

docs/css/main.css

@@ -146,3 +146,8 @@ ul.nav li.dropdown ul.dropdown-menu li.dropdown-submenu ul.dropdown-menu {
 .MathJax .mi { color: inherit }
 .MathJax .mf { color: inherit }
 .MathJax .mh { color: inherit }
+
+/**
+ * AnchorJS (anchor links when hovering over headers)
+ */
+a.anchorjs-link:hover { text-decoration: none; }

docs/js/main.js

@@ -68,46 +68,24 @@ function codeTabs() {
   });
 }
 
-function makeCollapsable(elt, accordionClass, accordionBodyId, title) {
-  $(elt).addClass("accordion-inner");
-  $(elt).wrap('<div class="accordion ' + accordionClass + '"></div>')
-  $(elt).wrap('<div class="accordion-group"></div>')
-  $(elt).wrap('<div id="' + accordionBodyId + '" class="accordion-body collapse"></div>')
-  $(elt).parent().before(
-    '<div class="accordion-heading">' +
-      '<a class="accordion-toggle" data-toggle="collapse" href="#' + accordionBodyId + '">' +
-             title +
-      '</a>' +
-    '</div>'
-  );
-}
-
-// Enable "view solution" sections (for exercises)
-function viewSolution() {
-  var counter = 0
-  $("div.solution").each(function() {
-    var id = "solution_" + counter
-    makeCollapsable(this, "", id,
-      '<i class="icon-ok-sign" style="text-decoration: none; color: #0088cc">' +
-      '</i>' + "View Solution");
-    counter++;
-  });
-}
 
 // A script to fix internal hash links because we have an overlapping top bar.
 // Based on https://github.com/twitter/bootstrap/issues/193#issuecomment-2281510
 function maybeScrollToHash() {
-  console.log("HERE");
   if (window.location.hash && $(window.location.hash).length) {
-    console.log("HERE2", $(window.location.hash), $(window.location.hash).offset().top);
     var newTop = $(window.location.hash).offset().top - 57;
     $(window).scrollTop(newTop);
   }
 }
 
 $(function() {
   codeTabs();
-  viewSolution();
+  // Display anchor links when hovering over headers. For documentation of the
+  // configuration options, see the AnchorJS documentation.
+  anchors.options = {
+    placement: 'left'
+  };
+  anchors.add();
 
   $(window).bind('hashchange', function() {
     maybeScrollToHash();