Reorganize user guide

Currently, the user guide is organized to cover just one method of usage. To add the second method methods of usage as described in #56, reorganize the user guide into a common introduction for using perun-node and move the existing guide to use perun-node as stand-alone application to a separate page.

Locate the current user guide and review issue #56 for the second usage method. Reorganize the guide around a shared introduction to using perun-node, then move the existing stand-alone application instructions to a separate page; done means both usage methods are covered without duplicating the common introduction.

Written by the indexing model from the issue text.