Template code snippets with `preview-component` don't render inside "demo" files

[didoo]

Similar to https://github.com/josemarluedke/docfy/issues/131 (I think)

When a template code snippet is included in a "demo" file, it's not rendered (as expected according to the documentation), just outputted.

I have created this mini-test:

and this is how it's rendered by Docfy: