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.