Mythic/.envfile you'll see which port you wan to run this on via the "DOCUMENTATION_PORT" key. You don't need to worry about this port too much since Mythic uses an Nginx reverse proxy to transparently proxy connections back to this container based on the web requests you make.
/Mythic/documentation-docker/folder and creates a static website from it. For development purposes, if you make changes to the markdown files here, the website will automatically update in real time. From the Mythic UI if you hit
/docs/then you'll be directed automatically to the main documentation home page.