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This document is superseded by the documentation in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): restore "make zipfile" target for pypi docs
fix(deisctl): correct typo in store-volume unit name
fix(docs): turn off "warnings as errors" for sphinx-build
fix(docs): move subheaders down a level for correct nav

The "PR Checklist" doc had all its subheaders at the same level as
the main header (h1 instead of h2), causing them to show up in the
right column navigation links.
Merge pull request #2290 from mboersma/docs-make-zipfile

fix(docs): restore "make zipfile" target for pypi docs
Merge pull request #2251 from bacongobbler/do-docs

feat(docs): add DigitalOcean provisioning guide; refactor installing_deis
Merge pull request #2296 from mboersma/fix-contrib-headers

fix(docs): move subheaders down a level for correct nav