- Edited quick start and tuning guide to simplify them a little
- Simplified top menu bar
- Made private a SparkContext constructor parameter that was left as
  public
- Various small fixes

also adding the version to the navbar so it is easy to tell which
version of Spark these docs were compiled for.

menu items that was causing the dropdowns to go away when the user
moved their mouse down towards them.

but removed the collapsable stuff from the nav bar.

responsive css (so nav menu doesn't switch to collapsed version
for narrow viewports).

easier to read.

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`.