- Add job scheduling docs
- Rename some fair scheduler properties
- Organize intro page better
- Link to Apache wiki for "contributing to Spark"

throughout the docs: SPARK_VERSION, SCALA_VERSION, and MESOS_VERSION.

To use them, e.g. use {{site.SPARK_VERSION}}.

Also removes uses of {{HOME_PATH}} which were being resolved to ""
by the templating system anyway.

which can be compiled via jekyll, using the command `jekyll`. To compile
and run a local webserver to serve the doc as a website, run
`jekyll --server`.