In the demo these fields are available;
But in the documentation they are nowhere to be found. Is there any documentation available?
Good question! The public-demo is a showcase of some of the ways you can customize a Payload project in a unique way. If you want to have anything in your project, just copy the code for it from the public-demo.
The SEO fields specifically are from this plugin: https://github.com/payloadcms/plugin-seo