Updates to installation instructions for Virtual Machine and Docker images

Staged at http://stg-web101.sw.joyent.continuuity.net/cdap/2.5.0/en/getstarted.html
Listed and created sections for the three different download methods.
Updated instructions for Virtual Machine and Docker.
The download page indicates nowhere that you are downloading a zip file. I am not sure whether it is clear. You have to click on the "(Download Symbol) SDK" button, and that will give you a zip file.