Add a functional documentation explaining available e-mail templates

The following documentation page explains how-to override e-mail templates and what e-mail templates are available by default: https://doc.nuxeo.com/nxdoc/how-to-customize-email-templates/

Yet, it seems we are lacking a functional explanation of the different templates, explaining for example in which cases is this e-mail template used.

Example:
publish.ftl: template used for sending an e-mail when a document is either published, approved or rejected, if the user has subscribed to this document or one of its parents.

modif.ftl: ...?

docPublishingSubject.ftl: ...?