Hmm I hadn't really considered being able to edit user guides within the CMS - I figured developers would be writing the user guides and I was just thinking of how to improve on what already exists rather than re-inventing the system.
What I was envisioning for the user guides is that the basics are automatically filled in from installed modules and updated when the modules are updated, and custom features are documented by developers in their own markdown files. It would be a good default for getting some basic and accurate user help in place without too much work.
User guides that are editable from within the CMS sounds like a great idea (basically a Wiki module) but I think automatically updating the user guides when modules are updated might be really difficult if all the text for the user guides is editable in the CMS. So it would probably require a greater user effort in entering the initial content and keeping it up to date over time.
I think it would be pretty hard to combine these two approaches so I think we're talking about two separate help systems - but maybe they could complement each other? So you could have two separate help items in the menu, maybe you call one 'User manual' which displays the developer-contributed 'standard' user guides, and you call the other one 'Tips' which shows user-contributed / client-specific Wiki content.
In that case I think only the Docsviewer approach should be included by default in Installer and Wiki could be optionally added on. You could edit your requirements to just have the Wiki though if you don't need the developer-contributed docs.