public_gemini folder is where you can place files to be hosted on your own Gemini subsection of dimension.sh. Any files placed in there will be available at the URL gemini://dimension.sh/~yourusername/.
The default page that'll be served if no filename is specified is index.gmi. Your public_gemini should already have an introductory file created for you as part of your new account. You can modify or remove this as needed. If no index.gmi is available for that folder, then the web server will generate a list of files.
The full spec for Gemini Text can be found here[1], but Flounder has a guide on Writing gemtext[2].
We're not restrictive of what can be hosted in your public_gemini files; we ask that you keep within the [[Rules]]. From time to time, the admins will check the contents of any public_gemini folder to make sure everything is above board, and they will email if they have any issues.
1: https://gitlab.com/gemini-specification/gemini-text/-/blob/master/specification.gmi
2: https://admin.flounder.online/gemini_text_guide.gmi
3: https://github.com/kr1sp1n/awesome-gemini