I don’t think there is a simple way to do this. Like you mentioned, in the Techdocs tutorial, an entity has to be created in order for the documentation to be displayed in the catalog.

This is the general philosophy of Backstage, where most of the plugins relies on entities. There are of course plugins that are « standalone » where you can benefits from there features without having to create an entity first. Like the Techradar plugin which display a page of ... a techradar.

So I think a good way to achieve your goal is to create your own front end plugin that will display a a page that is fetching via a backend plugin your markdown files (or anything) stored in a Git for example.

This way you could simply add your documentation to your Git repo and it will be updated by Backstage when you consult the page.

Here's some Backstage documentation that can help you with that :
- https://backstage.io/docs/plugins/create-a-plugin
- https://backstage.io/docs/plugins/backend-plugin