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 generates scala doc by calling `sbt/sbt doc`, copies it over
to docs, and updates the links from the api webpage to now point to
the copied over scaladoc (making the _site directory easy to just
copy over to a public website).

to use pygments syntax highlighting.