An existing page(main page) must have options to add side pages which can be used to have more information for a topic/content present on the main page.

Basically using the side pages as rough workspace to note down more info of a particular topic present on the main page where the side pages can be hidden when not needed.

Example:
1) An equation on the main page and its derivation on the side pages(collapsable/hidden pages) that can be accessed whenever required and can be hidden away when not required.

2)A formula on the main page and sample problems and solutions on the side pages that can be accessed whenever required and can be hidden away when not required.