REST_MakeProcess.C adding default documentation that needs to be updated by user

[jgalan]

Reminder. The updated macro generates a REST process skeleton that can be used by users to create new REST processes.

Changes proposed in this pull-request:

• It adds default documentation and documentation that must be filled by the user at the default generated process.

A new section at https://rest-for-physics.github.io/rest-advanced/process-development.html has been created to promote the use of this macro to generate templates.

It would be perhaps interesting to have another macro for pure metadata classes.

@rest-for-physics/core_dev