Sections for integrating perun-node into other third-party applications.

Perun node can be used in two different ways.

1. As a stand-alone application. This is intended way to use in applications with insufficient resources to run the perun-node locally. The third-party application can use the generate the client stubs for the API of perun-node and integrate it directly into their application. `perun-node cli` serves as a reference implementation to use the client stubs and also to quickly try using them.

2. As a library. This is intended way to use in applications with sufficient resources to run perun-node locally. The third-party application can directly use the payment API by importing the relevant packages from perun-node go module.

Currently, the user guide covers only part of the first method. Improve the section on first method (with steps to integrate the client stubs) and add sections for the second method.

This issue serves as umbrella issues for the sub tasks.

Start with the existing user-guide section covering the stand-alone integration method and compare it with the perun-node cli reference implementation. Document the steps for generating and using client stubs, then add a section explaining how to use the payment API through the perun-node Go module. Done means both integration methods are covered clearly for third-party application developers.

Written by the indexing model from the issue text.