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.
I know the initial reasons behind this flag and I am totally for it... it's just that I don't see another way to fix this :( Maybe with [nitpick_ignore](http://sphinx-doc.org/latest/invocation.html#cmdoption-sphinx-build-n)?