• _data/collapsible_examples.yml → use site.data.collapsible_examples
• _data/collapsible/examples.yml → use site.data.collapsible.examples

For your collapsible list include, in a markdown page you’d write:

include collapsible-list.html items=site.data.collapsible_examples.demo

or with folders:

include collapsible-list.html items=site.data.collapsible.examples.demo

If you want cleaner markdown, first assign it:

assign items = site.data.collapsible_examples.demo

include collapsible-list.html items=items

One important note with your current collapsible layout: it auto-reads only page.collapsible_items (front matter), not site.data.* directly. So to use _data without code changes, use the include call in page content as shown above.