"Getting Started" documentation rework

This is a rewrite of the "Getting Started" part of the website; it also replaces "Developing on Windows".

It covers the following:

* Setting up Windows and/or Linux for Crosswalk development.
* Setting up Tizen (emulated) and/or Android (real or emulated) as deployment targets.
* Building a very simple HTML5 application.
* Running the application on the target environments.

The docs should work with the current stable Crosswalk (version 3 at the time of writing).

It does not cover debugging, but https://crosswalk-project.org/jira/browse/XWALK-483 covers that.

Fixes https://crosswalk-project.org/jira/browse/XWALK-823