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.
This should probably get refactored into its own doc.