Document how to implement new CV/methods

[pabloferz]

Discussed in #179

^{Originally posted by sivadasetty August 1, 2022}
May be this is already considered but want to add it here. Wondering if we should list clearly the steps to implement a new CV or method in PySAGES user documentation. It would be nice to have a checklist especially on steps related to a) adding example systems, b) updating pysages tests, c) updating docs, and d) ci workflow.