Documentation for rest-client users

[renato-rg]

[francescoinfante]

Removed out of the epic for 2.9.

[francescoinfante]

@pastahito can you write what has been done and what is there to do?
nevermind this is for client documentation

[francescoinfante]

@davidrapin this would be part of an how to write plugins documentation epic.
is it for 2.10?