I’m not exactly a Gatsby expert and this template isn’t maintained by the Netlify CMS crew but here are some places to start @rulesstack :
You can find the menu code here. It’s mainly static so it shouldn’t be too hard to make changes.
To add new pages, you’ll have to create a markdown file here. If you need to edit the markdown files with Netlify CMS, you’ll have to reference them as files in the page collection in config.yml.