Jortel nodes userguide

The user-guide/nodes.rst explains nodes concepts and provides more of a *HowTo* perspective.  The command references are in admin-client/ and consumer-client/.

There may be some overlap but the goal and audience for the documents is different.

Since the consumer-client/nodes.rst will be mostly a cut-n-paste of the admin-client/nodes.rst, I'd like to get this reviewed first.  Then, I'll submit a separate pull request for the consumer.