Update developing plugin documentation

TODOs

• update the current docs to be in line with the current plugin protocol/functionalities
• generalize the current docs, removing Area C specificity
• add Area C specific descriptions / clarifications on the topic in the "domain-specific" section (this will be a combination of some of the current docs and some of the docs that we have in the Area C doc repo)

@ladinesa @pizarroj @ndaelman