[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