In last projects, here in communia we are using more and more the gitlab suite tools that are offering many free software projects. Calling projects for their own name: kde with its https://invent.kde.org , freedesktop with its https://gitlab.freedesktop.org , and where we are more active; https://gitlab.com and drupalcode.org https://git.drupalcode.org .
The schema that we are using is a broader group that includes the drupal recommended project and also where we are including the custom modules, themes, issues, and the wiki documentation. This way we can attach project references to each page, and also the wiki is part of the whole project system.
We found that when we are providing the project to the group(being a customer, and association or a social project), the content editor or the collaborator is a different person than the one who is reporting issues and has access to gitlab. So this person hasn't access to the forged wiki documentation. Of course that the wiki could be exported to pdf, hosted as mark-down with nextcloud, or served using ruby with gollum. But it will be detached of your final shiny drupal product.
So here comes this simple module: https://www.drupal.org/project/git_wiki_help . With it, you could add the exported or cloned git wiki to a directory in (private) files path. And then it will be accessible in native drupal administrator help pages, giving the person who administrates the web the general project documentation.
Take note that wiki is not updated in real-time as if there's an update in git wiki it needs to manually pulled (or If using git you could hang to git hooks to update automatically).