docs(test_plan): add a formal doc describing QA strategy & scope
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.
fix(docs): fixup markdown
docs(understanding_deis): refactor architecture, concepts, components
Merge pull request #2448 from mboersma/test-plan-doc

docs(test_plan): add a formal doc describing QA strategy & scope
Merge pull request #2447 from bacongobbler/2446-ssh-key-docs

docs(installing_deis): add quick start guide
Merge pull request #2450 from gabrtv/understanding-deis

docs(understanding_deis): refactor architecture, concepts, components