[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 <joshrosen@databricks.com>

Closes #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

1 files changed, 5 insertions, 0 deletions

diff --git a/docs/css/main.css b/docs/css/main.css
index f6fe7d5f07..89305a7d3a 100755
--- a/docs/css/main.css
+++ b/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; }