docs(installing_deis): add quick start guide

This guide adds some quick links to some common tasks that are shared
across multiple cloud provider documentation such as generating a new
SSH key, generating a Discovery URL, configuring DNS and installing
deisctl.

closes #2446
Good point. I made a link later on that shows how to install the platform with `deisctl` which has an important sidenote to install `deisctl` beforehand. It would probably be better to put up a link for installing deisctl here.