Add Expandable/Collapsible Sections

This is similar to GitHub's \<details>\<summary> blocks What is it? The ability to have an expandable section within a document. The expandable area would have all the same formatting and embedding support (minus another expandable section). The expandable area will have a title to identify the content at a glance. This section can be toggled open to expose the inner article contents when viewing the doc. The section, by default, should be closed. The option to have it open by default should also be available. Why is this useful? An expandable section would allow extra data or details to be stored in a doc without taking up visual real estate. One case could be a document about the workings of a Program's Class with an expandable section giving examples of various Methods within the class. Another case could be the expected output of a process in a tutorial. Current Alternatives The only alternative is to embed another document into the current …