feat(docs): add digitalocean guide
ref(docs): shift management docs, add provider-specific docs
ref(docs): remove "register admin user" docs

This document is superseded by the documentation in
using_deis/install-client and in using_deis/register-user.
ref(docs): add link to installing Deis
fix(docs): add internal links for installation guides
ref(docs): add installation docs for deisctl
ref(docs): move DNS hints into sidenotes
fix(docs): turn off "warnings as errors" for sphinx-build
Merge pull request #2251 from bacongobbler/do-docs

feat(docs): add DigitalOcean provisioning guide; refactor installing_deis