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`.
