BookStack

BookStack organizes documentation the way non-wiki-people think: shelves hold books, books hold chapters and pages. That fixed hierarchy is a constraint by design, and it is exactly why teams that bounced off free-form wikis stick with BookStack.

It is a PHP/Laravel app with a WYSIWYG-or-Markdown editor, solid search, per-shelf permissions and painless upgrades. Diagrams.net integration handles the architecture drawings.

The go-to recommendation for internal documentation where "where does this page go?" should never be a debate.