For many previous versions of Drupal, modules have been able to provide module-level overview help topics by implementing hook_help().
This is still possible to do in Drupal 8, but there is an additional method for providing help topics now, with the Help Topics experimental module. This module allows modules, themes, and profiles to provide one or more help topics. Topics are distributed as plugins. Each topic is defined in a Twig file, which contains a few settings in a "front matter" section at the top (topic title, "top-level", and "related", see below), as well as the text/HTML body of the topic. These go into a sub-directory called "help_topics" in the module, theme, or profile directory. See the topics in core/modules/help_topics/help_topics to use as examples, and the Help topic standards page for more details on how to write good topics.
