Document what a `.podium` file is

### What is the problem or limitation you are having?

I don't know what a `.podium` file represents.

### Describe the solution you'd like

To know how to make a `.podium` file.

### Describe alternatives you've considered

Not caring and just working with a `.md` file directly only.

### Additional context

The file picker suggests this file type is a thing.

Start by locating the project's existing documentation or examples that mention `.podium` files, then compare them with the Markdown workflow described in the issue. Document what a `.podium` file represents and how to create one; done means a newcomer can understand the format and begin using it without relying on guesswork.

Written by the indexing model from the issue text.