Once you have gotten up and running, there will be lots of the demo content populated. The following files and directories welcome (need) your customization:
/src/docs Put your Markdown files in here./src/docs/table_of_contents.json Be sure that the structure of this is kept in sync with your Markdown files./src/styles/globalStyles.js The styled-components tool for setting global styles./src/styles/theme.js The styled-components theme used throughout the site./src/styles/css/code.css The PrismJS for code block syntax highlighting./src/images Add your images here (note that starter images are queried, so query would need to be removed or image replaced)./src/pages Add any pages you want here. You can make them pure React, or build another Markdown based page./src/demo This page can be completely rewritten./src/components Design and tweak this as you see fit (note that the <TableOfContents / > component is important so be careful with its functionality).Other files like gatsby-node.js and gatsby-config.js are certainly customizable. Best idea is to familiarize yourself with how it works first.
There is a lot of content and a few npm packages from the Demo page that you won't need. Run npm run fresh to uninstall these and reset the Demo.jsx page. The npm run fresh command can then be deleted from the package.json. You will probably need to restart gatsby develop after this.