docs(managing_deis): provide login instructions

Add a page to the docs to explicitly define how to connect to a cluster.

ref #2523
When we refer to an example IP address, we should always use one from [RFC5737](https://tools.ietf.org/html/rfc5737) rather than a real one. (We should probably pick a few addresses from there to use consistently in our docs, if we don't already.)