Raneto is an open-source nodejs knowledge base powered by markdown. It's very lightweight, and I currently have it hosted at wiki.jakegines.in.
Heroku is the best free method of hosting Raneto that I have found so far. Raneto provides a short guide on heroku hosting here. Unfortunately, Heroku uses an Ephermeral Disk, meaning any changes written to the disk will not persist after the application is restarted. I am currently looking for a way around this.
As of 5/15/2022, users looking to use Raneto should implement this PR manually. I emailed the primary maintainer about this a while back, but so far he hasn't responded.