How to Add to MkDocs
Where to Find it?
If you go to this link, you can either clone the repository or manually edit the markdown files using gitlab's editor.
Note
To have the markdown files show up on the side, you need to edit the mkdocs.yml file and add your markdown file to the nav section. The file can be found at build/mkdocs.yml. If this is not done the page is still accessible through links from other files and manually changing the url, but it will not show up on the navigation on the left side of the page.
Formatting
Largely it is up to your discretion how things are done. In the future it may be a good idea to come up with templates or a better guide on how to start, but I believe this will be enough information for someone to begin contributing.
What You Can Do
MkDocs uses the Python-Markdown implmentation of markdown. Which means there may be some differences in syntax but all the basics will be the same. Use this link to see some examples of what you can do in markdown.