We use a git repository full of markdown files for out team's documentation and it works really well for us.

Benefits include:

* It's really simple for people to use and update.

* Everyone can keep a copy locally on their own machines in case they're stuck without access to the GitHub/GitLab instance

* People are much happier removing out of date information from it as they can always find in the history again if they need it

* Branches in documentation can be really useful when you're making big changes (or an individual wants to add personal comments to their local copy)