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.
So it is one of the dash characters in this text, it is not a dash, but some other similar-looking non-ASCII character. Can you make sure only to use ASCII here.