#3037230 - Moves Help Topics hook_help() to help_help() and to new topic help.overview.html.twig in core Help and deletes help_topic.module.

#3037230 - Adds new help topic in core Help module to replace Help Topic module overview in its hook_help. Moves help.help_topic route case to help_help(). Deletes help_topic.module, as all other hooks are already being moved in #3087499.