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

This refactors the `Installing Deis` section of the documentation to list off all of the supported providers, similar to https://docs.docker.com/installation/, as well as adding a DigitalOcean guide as a first-class citizen in the documentation. I've moved over other documentation living under "Installing Deis" such as configuring SSL endpoints and the like to "Managing Deis" as it better fits there.
If we disable warnings, we can't detect broken links or other doc errors. The doc build will essentially never fail, which is why I added the `-W` flag in the first place: Sphinx is stupid.