[SPARK-5608] Improve SEO of Spark documentation pages

- Add meta description tags on some of the most important doc pages
- Shorten the titles of some pages to have more relevant keywords; for
  example there's no reason to have "Spark SQL Programming Guide - Spark
  1.2.0 documentation", we can just say "Spark SQL - Spark 1.2.0
  documentation".

Author: Matei Zaharia <matei@databricks.com>

Closes #4381 from mateiz/docs-seo and squashes the following commits:

4940563 [Matei Zaharia] [SPARK-5608] Improve SEO of Spark documentation pages

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/_layouts/global.html b/docs/_layouts/global.html
index 8841f7675d..efc4c61293 100755
--- a/docs/_layouts/global.html
+++ b/docs/_layouts/global.html
@@ -7,7 +7,9 @@
         <meta charset="utf-8">
         <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
         <title>{{ page.title }} - Spark {{site.SPARK_VERSION_SHORT}} Documentation</title>
-        <meta name="description" content="">
+        {% if page.description %}
+          <meta name="description" content="{{page.description | replace: 'SPARK_VERSION_SHORT', site.SPARK_VERSION_SHORT}}">
+        {% endif %}
 
         {% if page.redirect %}
           <meta http-equiv="refresh" content="0; url={{page.redirect}}">