Collapsible list item: New demo: Fixes #5539 - Collapsible list items
Demos: Collapsible list items: Simplifying examples.
Demos: Added collapsible list items demo to the menu
Demos: Collapsible list item: Making whitespace consistent.
Demos: Collapsible list items: Splitting into two demos: flat and indented.
Demos: Collapsible list items: Adding example of list item that does not itself contain another <ul> and cleaning up styling.
Demos: Navigation: Reflect collapsible list item demo split.
Demos: Collapsible list item: Indentation changes.