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
Before "Choose a Provider", this would also be a great place to call out and link to installing deisctl. I'm also wondering if we should make this the index page for Installing Deis rather than a separate page.